TypeInitializationException: The type initializer for 'System.Data.Entity.Internal.AppConfig' threw an exception

I am currently trying to create an ASP.NET Core API to an application called "Application". It contains customers and products. When I make a request to get all customers through my CustomerController I get the following error:

System.TypeInitializationException
HResult=0x80131534
Message=The type initializer for 'System.Data.Entity.Internal.AppConfig' threw an exception.
Source=EntityFramework
StackTrace:
at System.Data.Entity.Internal.LazyInternalConnection..ctor(DbContext context, 
String nameOrConnectionString)
at System.Data.Entity.DbContext..ctor(String nameOrConnectionString)
at Application.TestAPI.Logic.Database.ApplicationDatabaseContext..ctor(String connString) in C:\Users\Chris\Source\repos\Application.TestAPI\Application.TestAPI.Logic\Database\ApplicationDatabaseContext.cs:line 9
at Application.TestAPI.Logic.Init.<>c__DisplayClass0_0.<AddLogic>b__0(IServiceProvider _) in C:\Users\Chris\Source\repos\Application.TestAPI\Application.TestAPI.Logic\Init.cs:line 19
at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitFactory(FactoryCallSite factoryCallSite, ServiceProviderEngineScope scope)
at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(IServiceCallSite callSite, TArgument argument)
at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScoped(ScopedCallSite scopedCallSite, ServiceProviderEngineScope scope)
at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(IServiceCallSite callSite, TArgument argument)
at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitConstructor(ConstructorCallSite constructorCallSite, ServiceProviderEngineScope scope)
at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(IServiceCallSite callSite, TArgument argument)
at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScoped(ScopedCallSite scopedCallSite, ServiceProviderEngineScope scope)
at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(IServiceCallSite callSite, TArgument argument)
at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitConstructor(ConstructorCallSite constructorCallSite, ServiceProviderEngineScope scope)
at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(IServiceCallSite callSite, TArgument argument)
at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScoped(ScopedCallSite scopedCallSite, ServiceProviderEngineScope scope)
at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(IServiceCallSite callSite, TArgument argument)
at Microsoft.Extensions.DependencyInjection.ServiceLookup.DynamicServiceProviderEngine.<>c__DisplayClass1_0.<RealizeService>b__0(ServiceProviderEngineScope scope)
at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.GetService(Type serviceType, ServiceProviderEngineScope serviceProviderEngineScope)
at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScope.GetService(Type serviceType)
at Microsoft.Extensions.DependencyInjection.ActivatorUtilities.GetService(IServiceProvider sp, Type type, Type requiredBy, Boolean isDefaultParameterRequired)
at Microsoft.AspNetCore.Mvc.Controllers.ControllerActivatorProvider.<>c__DisplayClass4_0.<CreateActivator>b__0(ControllerContext controllerContext)
at Microsoft.AspNetCore.Mvc.Controllers.ControllerFactoryProvider.<>c__DisplayClass5_0.<CreateControllerFactory>g__CreateController|0(ControllerContext controllerContext)
at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.<InvokeInnerFilterAsync>d__13.MoveNext()
Inner Exception 1:
FileNotFoundException: Could not load file or assembly 
'System.Configuration.ConfigurationManager, Version=0.0.0.0, Culture=neutral, 
PublicKeyToken=cc7b13ffcd2ddd51'. Det går inte att hitta filen.

The code/class where it crashes is:

using Application.TestAPI.Logic.Models;
using System.Data.Entity;
using System.Diagnostics;

namespace Application.TestAPI.Logic.Database
{
    public class ApplicationDatabaseContext : DbContext
    {
        public ApplicationDatabaseContext(string connString) : base(connString)
        {
           //Database.Log = s => Debug.WriteLine(s);
        }

    public DbSet<ApplicationCustomer> Customers { get; set; }
    public DbSet<ApplicationContract> Contracts { get; set; }
    }
}

The connectionstring is not placed in my App.config for the classlibrary but rather in my appsettings.json in the API-project.

The parameter connString is a valid connectionstring that I know works and is passed from the following line of code:

services.AddScoped(_ => new ApplicationDatabaseContext(applicationConfiguration.GetValue<string>("ConnectionString"))); 

My *.csproj-file looks as follows:

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="15.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
  <PropertyGroup>
    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
    <ProjectGuid>{6347E02C-B759-404B-8E2F-20B67E2524B2}</ProjectGuid>
    <OutputType>Library</OutputType>
    <AppDesignerFolder>Properties</AppDesignerFolder>
    <RootNamespace>Application.TestAPI.Logic</RootNamespace>
    <AssemblyName>Application.TestAPI.Logic</AssemblyName>
    <TargetFrameworkVersion>v4.6.2</TargetFrameworkVersion>
    <FileAlignment>512</FileAlignment>
    <Deterministic>true</Deterministic>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
    <DebugSymbols>true</DebugSymbols>
    <DebugType>full</DebugType>
    <Optimize>false</Optimize>
    <OutputPath>bin\Debug\</OutputPath>
    <DefineConstants>DEBUG;TRACE</DefineConstants>
    <ErrorReport>prompt</ErrorReport>
    <WarningLevel>4</WarningLevel>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
    <DebugType>pdbonly</DebugType>
    <Optimize>true</Optimize>
    <OutputPath>bin\Release\</OutputPath>
    <DefineConstants>TRACE</DefineConstants>
    <ErrorReport>prompt</ErrorReport>
    <WarningLevel>4</WarningLevel>
  </PropertyGroup>
  <ItemGroup>
    <Reference Include="EntityFramework, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089, processorArchitecture=MSIL">
      <HintPath>..\packages\EntityFramework.6.2.0\lib\net45\EntityFramework.dll</HintPath>
    </Reference>
    <Reference Include="EntityFramework.SqlServer, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089, processorArchitecture=MSIL">
      <HintPath>..\packages\EntityFramework.6.2.0\lib\net45\EntityFramework.SqlServer.dll</HintPath>
    </Reference>
    <Reference Include="Microsoft.EntityFrameworkCore">
      <HintPath>..\..\..\..\..\..\Program Files\dotnet\sdk\NuGetFallbackFolder\microsoft.entityframeworkcore\2.1.1\lib\netstandard2.0\Microsoft.EntityFrameworkCore.dll</HintPath>
    </Reference>
    <Reference Include="Microsoft.Extensions.Configuration, Version=2.1.1.0, Culture=neutral, PublicKeyToken=adb9793829ddae60, processorArchitecture=MSIL">
      <HintPath>..\packages\Microsoft.Extensions.Configuration.2.1.1\lib\netstandard2.0\Microsoft.Extensions.Configuration.dll</HintPath>
    </Reference>
    <Reference Include="Microsoft.Extensions.Configuration.Abstractions, Version=2.1.1.0, Culture=neutral, PublicKeyToken=adb9793829ddae60, processorArchitecture=MSIL">
      <HintPath>..\packages\Microsoft.Extensions.Configuration.Abstractions.2.1.1\lib\netstandard2.0\Microsoft.Extensions.Configuration.Abstractions.dll</HintPath>
    </Reference>
    <Reference Include="Microsoft.Extensions.Configuration.Binder, Version=2.1.1.0, Culture=neutral, PublicKeyToken=adb9793829ddae60, processorArchitecture=MSIL">
      <HintPath>..\packages\Microsoft.Extensions.Configuration.Binder.2.1.1\lib\netstandard2.0\Microsoft.Extensions.Configuration.Binder.dll</HintPath>
    </Reference>
    <Reference Include="Microsoft.Extensions.DependencyInjection.Abstractions, Version=2.1.1.0, Culture=neutral, PublicKeyToken=adb9793829ddae60, processorArchitecture=MSIL">
      <HintPath>..\packages\Microsoft.Extensions.DependencyInjection.Abstractions.2.1.1\lib\netstandard2.0\Microsoft.Extensions.DependencyInjection.Abstractions.dll</HintPath>
    </Reference>
    <Reference Include="Microsoft.Extensions.Options, Version=2.1.1.0, Culture=neutral, PublicKeyToken=adb9793829ddae60, processorArchitecture=MSIL">
      <HintPath>..\packages\Microsoft.Extensions.Options.2.1.1\lib\netstandard2.0\Microsoft.Extensions.Options.dll</HintPath>
    </Reference>
    <Reference Include="Microsoft.Extensions.Options.ConfigurationExtensions, Version=2.1.1.0, Culture=neutral, PublicKeyToken=adb9793829ddae60, processorArchitecture=MSIL">
      <HintPath>..\packages\Microsoft.Extensions.Options.ConfigurationExtensions.2.1.1\lib\netstandard2.0\Microsoft.Extensions.Options.ConfigurationExtensions.dll</HintPath>
    </Reference>
    <Reference Include="Microsoft.Extensions.Primitives, Version=2.1.1.0, Culture=neutral, PublicKeyToken=adb9793829ddae60, processorArchitecture=MSIL">
      <HintPath>..\packages\Microsoft.Extensions.Primitives.2.1.1\lib\netstandard2.0\Microsoft.Extensions.Primitives.dll</HintPath>
    </Reference>
    <Reference Include="Application.Core, Version=6.6.2.0, Culture=neutral, processorArchitecture=MSIL">
      <HintPath>..\packages\Application.Core.6.6.2\lib\net46\Application.Core.dll</HintPath>
    </Reference>
    <Reference Include="Application.Core.Dto, Version=1.0.0.0, Culture=neutral, processorArchitecture=MSIL">
      <HintPath>..\packages\Application.Core.6.6.2\lib\net46\Application.Core.Dto.dll</HintPath>
    </Reference>
    <Reference Include="Newtonsoft.Json, Version=11.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed, processorArchitecture=MSIL">
      <HintPath>..\packages\Newtonsoft.Json.11.0.2\lib\net45\Newtonsoft.Json.dll</HintPath>
    </Reference>
    <Reference Include="StructureMap">
      <HintPath>Lib\StructureMap.dll</HintPath>
    </Reference>
    <Reference Include="StructureMap.Net4">
      <HintPath>Lib\StructureMap.Net4.dll</HintPath>
    </Reference>
    <Reference Include="StructureMap.Web">
      <HintPath>Lib\StructureMap.Web.dll</HintPath>
    </Reference>
    <Reference Include="System" />
    <Reference Include="System.Buffers, Version=4.0.3.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51, processorArchitecture=MSIL">
      <HintPath>..\packages\System.Buffers.4.5.0\lib\netstandard2.0\System.Buffers.dll</HintPath>
    </Reference>
    <Reference Include="System.ComponentModel.DataAnnotations" />
    <Reference Include="System.Configuration" />
    <Reference Include="System.Core" />
    <Reference Include="System.Data.OracleClient" />
    <Reference Include="System.Drawing" />
    <Reference Include="System.IO.Pipelines, Version=4.0.0.1, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51, processorArchitecture=MSIL">
      <HintPath>..\packages\System.IO.Pipelines.4.5.2\lib\netstandard2.0\System.IO.Pipelines.dll</HintPath>
    </Reference>
    <Reference Include="System.Memory, Version=4.0.1.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51, processorArchitecture=MSIL">
      <HintPath>..\packages\System.Memory.4.5.1\lib\netstandard2.0\System.Memory.dll</HintPath>
    </Reference>
    <Reference Include="System.Net" />
    <Reference Include="System.Numerics" />
    <Reference Include="System.Numerics.Vectors, Version=4.1.4.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL">
      <HintPath>..\packages\System.Numerics.Vectors.4.5.0\lib\net46\System.Numerics.Vectors.dll</HintPath>
    </Reference>
    <Reference Include="System.Runtime.CompilerServices.Unsafe, Version=4.0.4.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL">
      <HintPath>..\packages\System.Runtime.CompilerServices.Unsafe.4.5.1\lib\netstandard2.0\System.Runtime.CompilerServices.Unsafe.dll</HintPath>
    </Reference>
    <Reference Include="System.Security.AccessControl, Version=4.1.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL">
      <HintPath>..\packages\System.Security.AccessControl.4.5.0\lib\net461\System.Security.AccessControl.dll</HintPath>
    </Reference>
    <Reference Include="System.Security.Permissions, Version=4.0.1.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51, processorArchitecture=MSIL">
      <HintPath>..\packages\System.Security.Permissions.4.5.0\lib\net461\System.Security.Permissions.dll</HintPath>
    </Reference>
    <Reference Include="System.Security.Principal.Windows, Version=4.1.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL">
      <HintPath>..\packages\System.Security.Principal.Windows.4.5.0\lib\net461\System.Security.Principal.Windows.dll</HintPath>
    </Reference>
    <Reference Include="System.ServiceProcess" />
    <Reference Include="System.Threading.Tasks.Extensions, Version=4.2.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51, processorArchitecture=MSIL">
      <HintPath>..\packages\System.Threading.Tasks.Extensions.4.5.1\lib\netstandard2.0\System.Threading.Tasks.Extensions.dll</HintPath>
    </Reference>
    <Reference Include="System.Transactions" />
    <Reference Include="System.Xml.Linq" />
    <Reference Include="System.Data.DataSetExtensions" />
    <Reference Include="Microsoft.CSharp" />
    <Reference Include="System.Data" />
    <Reference Include="System.Net.Http" />
    <Reference Include="System.Xml" />
  </ItemGroup>
  <ItemGroup>
    <Compile Include="Configuration\ApplicationOptions.cs" />
    <Compile Include="Connector\ApplicationConnector.cs" />
    <Compile Include="Database\CustomerConfiguration.cs" />
    <Compile Include="Database\ApplicationDatabaseContext.cs" />
    <Compile Include="Init.cs" />
    <Compile Include="Models\Customer.cs" />
    <Compile Include="Models\CustomerDto.cs" />
    <Compile Include="Models\Filter.cs" />
    <Compile Include="Models\ApplicationContract.cs" />
    <Compile Include="Models\ApplicationCustomer.cs" />
    <Compile Include="Properties\AssemblyInfo.cs" />
    <Compile Include="Services\CustomerService.cs" />
    <Compile Include="Services\FilterService.cs" />
    <Compile Include="Services\ICustomerService.cs" />
    <Compile Include="Services\AppliationCustomerService.cs" />
    <Compile Include="Translators\CustomerTranslator.cs" />
    <Compile Include="Translators\ApplicationCustomerTranslator.cs" />
  </ItemGroup>
  <ItemGroup>
    <None Include="app.config">
      <SubType>Designer</SubType>
    </None>
    <None Include="packages.config" />
  </ItemGroup>
  <ItemGroup />
  <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
</Project>

I have tried re-arranging my App.config, reinstalling EntityFramework, hard-coding my connectionstring and installing System.Configuration.ConfigurationManager through NuGet. Nothing has worked so far.

Any suggestions/explanations on what might be causing this?

Regards,

Chris