Untouched code suddenly throws Method not found: 'Void Swashbuckle.Application.SwaggerDocsConfig.RootUrl' on startup

I just opened an application of which I need it's API to create the necessary data required for the application I'm actually working on.

On startup of the API I get this error:

System.MissingMethodException
  HResult=0x80131513
  Message=Method not found: 'Void Swashbuckle.Application.SwaggerDocsConfig.RootUrl(System.Func`2<System.Net.Http.HttpRequestMessage,System.String>)'.
  Source=Scanner.Api
  StackTrace:
   at Scanner.Api.Configuration.SwaggerConfiguration.<>c.<ConfigureApiDocumentation>b__0_0(SwaggerDocsConfig c) in C:\MyApplication.Api\Configuration\SwaggerConfiguration.cs:line 176
   at Swashbuckle.Application.HttpConfigurationExtensions.EnableSwagger(HttpConfiguration httpConfig, String routeTemplate, Action`1 configure)
   at Scanner.Api.Configuration.SwaggerConfiguration.ConfigureApiDocumentation(HttpConfiguration httpConfiguration) in C:\MyApplication.Api\Configuration\SwaggerConfiguration.cs:line 13
   at Scanner.Api.Startup.Configuration(IAppBuilder app, IdentityServerBearerTokenAuthenticationOptions identityServerBearerTokenAuthenticationOptions) in C:\MyApplication.Api\Startup.cs:line 61

The only thing that's changed is that my laptop has had a full Windows reinstall.

The code itself was cloned and no changes.

The SwaggerConfiguration mentioned above:

public static class SwaggerConfiguration
{
    public static HttpConfiguration ConfigureApiDocumentation(this HttpConfiguration httpConfiguration)
    {
        httpConfiguration
            .EnableSwagger("docs/{apiVersion}/swagger", c =>
            {
                c.RootUrl(RootUrlResolver);
                c.SingleApiVersion("v1", "Scanner.Api");
                var xmlCommentsFile = XmlCommentsFilePath();
                if (File.Exists(xmlCommentsFile))
                    c.IncludeXmlComments(xmlCommentsFile);

                c.DescribeAllEnumsAsStrings();
            })
            .EnableSwaggerUi("docs/{*assetPath}", c =>
            {
                c.DisableValidator();
            });
        return httpConfiguration;
    }

    private static string XmlCommentsFilePath()
    {
        var xmlCommentsPath = $"{AppDomain.CurrentDomain.BaseDirectory}\\bin\\Scanner.Api.xml";
        return xmlCommentsPath;
    }


    private static string RootUrlResolver(HttpRequestMessage request)
    {
        var rootUrl = $"{request.RequestUri.GetLeftPart(UriPartial.Authority)}{request.GetRequestContext().VirtualPathRoot.TrimEnd('/')}";
        return rootUrl;
    }
}

What could cause this and what's the solution?

Update

I did an update-package to force every package to the latest version, followed by a delete of all bin and obj folders in the entire solution structure. Didn't fix the issue.

Try to recompile all your code and ensure that all the .dll files are the correct ones into the "bin" directory.