Validation of viewstate MAC failed. No changes to code in at least a month

I was made aware recently of a "Validation of viewstate MAC failed" error on my website.

Specifics are:

• We are using a single website that is on-site, so it should not be a cluster issue.
• No code change in over a month
• The Error pops up in Chrome and Edge. IE and Firefox give a different issue.
• Server OS is server 2008
• According to IIS Manager we are using IIS7
• Defining the machine keys gave no change
• Adding the debug code suggested on the error page resulted in a 500 error
• DotNet 4.8.03752 (from VS help>about)
• Visual Studio 17 15.9.29 (from VS help>about)
• Running the code from inside VS produces no errors
• key options for both Validation Key and Decryption Key section in IIS are:
  • Automatically generate at runtime
  • Generate a unique key for each application

Walkthrough:

• Load Webpage
• click "Login"
• Insert username and password
• submit
• receive error. On firefox and IE it continues through without crashing but acts as if the username does not exist.

We have tried:

• inserting Machine Keys into the code
• adding both options for debugging
• removing updates to the server's OS
• Updating the server's OS
• Digging through code that might be relevant
• reverting to code from 3 months ago
• Working on migrating to a newer server OS and IIS

Web.config (relevant parts based on other questions)

    <system.web>
<compilation debug="true" targetFramework="4.5">
  <assemblies>
    <add assembly="System.Design, Version=4.0.0.0, Culture=neutral, PublicKeyToken=..." />
    <add assembly="System.Web.Extensions.Design, Version=4.0.0.0, Culture=neutral, PublicKeyToken=..." />
    <add assembly="System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=..." />
    <add assembly="System.Speech, Version=4.0.0.0, Culture=neutral, PublicKeyToken=..." />
    <add assembly="Microsoft.Build.Framework, Version=4.0.0.0, Culture=neutral, PublicKeyToken=..." />
    <add assembly="System.Web.DataVisualization, Version=4.0.0.0, Culture=neutral, PublicKeyToken=..." />
    <add assembly="Microsoft.ReportViewer.WebForms, Version=12.0.0.0, Culture=neutral, PublicKeyToken=..." />
    <add assembly="Microsoft.ReportViewer.Common, Version=12.0.0.0, Culture=neutral, PublicKeyToken=..." />
  </assemblies>
  <buildProviders>
    <add extension=".rdlc" type="Microsoft.Reporting.RdlBuildProvider, Microsoft.ReportViewer.WebForms, Version=12.0.0.0, Culture=neutral, PublicKeyToken=..." />
  </buildProviders>
</compilation>
<httpRuntime targetFramework="4.5" />
<pages>
  <namespaces>
    <add namespace="System.Web.Optimization" />
  </namespaces>
  <controls>
    <add tagPrefix="webopt" namespace="Microsoft.AspNet.Web.Optimization.WebForms" assembly="Microsoft.AspNet.Web.Optimization.WebForms" />
    <add tagPrefix="telerik" namespace="Telerik.Web.UI" assembly="Telerik.Web.UI" />
    <add tagPrefix="asp" namespace="System.Web.UI.DataVisualization.Charting" assembly="System.Web.DataVisualization, Version=4.0.0.0, Culture=neutral, PublicKeyToken=..." />
  </controls>
</pages>
<authentication mode="Forms">
  <!--<forms loginUrl="~/Login" timeout="2880" defaultUrl="~/" />-->
</authentication>
<profile defaultProvider="DefaultProfileProvider">
  <providers>
    <add name="DefaultProfileProvider" type="System.Web.Providers.DefaultProfileProvider, System.Web.Providers, Version=2.0.0.0, Culture=neutral, PublicKeyToken=..." connectionStringName="DefaultConnection" applicationName="/" />
  </providers>
</profile>
<membership defaultProvider="DefaultMembershipProvider">
  <providers>
    <add name="DefaultMembershipProvider" type="System.Web.Providers.DefaultMembershipProvider, System.Web.Providers, Version=2.0.0.0, Culture=neutral, PublicKeyToken=..." connectionStringName="DefaultConnection" enablePasswordRetrieval="false" enablePasswordReset="true" requiresQuestionAndAnswer="false" requiresUniqueEmail="false" maxInvalidPasswordAttempts="5" minRequiredPasswordLength="6" minRequiredNonalphanumericCharacters="0" passwordAttemptWindow="10" applicationName="/" />
  </providers>
</membership>
<roleManager defaultProvider="DefaultRoleProvider">
  <providers>
    <add name="DefaultRoleProvider" type="System.Web.Providers.DefaultRoleProvider, System.Web.Providers, Version=2.0.0.0, Culture=neutral, PublicKeyToken=..." connectionStringName="DefaultConnection" applicationName="/" />
  </providers>
</roleManager>
<!--
        comments
  -->
<sessionState mode="InProc" customProvider="DefaultSessionProvider">
  <providers>
    <add name="DefaultSessionProvider" type="System.Web.Providers.DefaultSessionStateProvider, System.Web.Providers, Version=2.0.0.0, Culture=neutral, PublicKeyToken=..." connectionStringName="DefaultConnection" />
  </providers>
</sessionState>
<httpHandlers>
  <add path="Telerik.Web.UI.WebResource.axd" verb="*" type="Telerik.Web.UI.WebResource" validate="false" />
  <add path="ChartImage.axd" verb="*" type="Telerik.Web.UI.ChartHttpHandler" validate="false" />
  <add path="Telerik.Web.UI.SpellCheckHandler.axd" verb="*" type="Telerik.Web.UI.SpellCheckHandler" validate="false" />
  <add path="Telerik.Web.UI.DialogHandler.aspx" verb="*" type="Telerik.Web.UI.DialogHandler" validate="false" />
  <add path="Telerik.RadUploadProgressHandler.ashx" verb="*" type="Telerik.Web.UI.RadUploadProgressHandler" validate="false" />
  <add path="ChartImg.axd" verb="GET,HEAD,POST" type="System.Web.UI.DataVisualization.Charting.ChartHttpHandler, System.Web.DataVisualization, Version=4.0.0.0, Culture=neutral, PublicKeyToken=..." validate="false" />
  <add path="Reserved.ReportViewerWebControl.axd" verb="*" type="Microsoft.Reporting.WebForms.HttpHandler, Microsoft.ReportViewer.WebForms, Version=12.0.0.0, Culture=neutral, PublicKeyToken=..." validate="false" />
</httpHandlers>
<httpModules />
<customErrors mode="Off"></customErrors>

<compilation debug="true"/>

</system.web>

Stacktrace

[ViewStateException: Invalid viewstate. 
    Client IP: Redacted
    Port: Redacted
    Referer: Redacted/Login
    Path: Redacted/Login
    User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36
    ViewState: A5c...]

[HttpException (0x80004005): Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.

See http://go.microsoft.com/fwlink/?LinkID=314055 for more information.]
   System.Web.UI.ViewStateException.ThrowError(Exception inner, String persistedState, String errorPageMessage, Boolean macValidationError) +124
   System.Web.UI.ObjectStateFormatter.Deserialize(String inputString, Purpose purpose) +444
   System.Web.UI.ObjectStateFormatter.System.Web.UI.IStateFormatter2.Deserialize(String serializedState, Purpose purpose) +14
   System.Web.UI.Util.DeserializeWithAssert(IStateFormatter2 formatter, String serializedState, Purpose purpose) +41
   System.Web.UI.HiddenFieldPageStatePersister.Load() +255
   System.Web.UI.Page.LoadPageStateFromPersistenceMedium() +257
   System.Web.UI.Page.LoadAllState() +36
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +6796
   System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +250
   System.Web.UI.Page.ProcessRequest() +76
   System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context) +24
   System.Web.UI.Page.ProcessRequest(HttpContext context) +62
   ASP.account_login_aspx.ProcessRequest(HttpContext context) +4
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +195
   System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step) +50
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +73

Page Code

    <%@ Page Title="Log in" Language="C#" MasterPageFile="~/Site.Master" AutoEventWireup="true" CodeBehind="Login.aspx.cs" Inherits="CCD.Account.Login" %>
<%@ Register Src="~/Account/OpenAuthProviders.ascx" TagPrefix="uc" TagName="OpenAuthProviders" %>
<%@ Page Language="C#" Debug="true" %>

<asp:Content runat="server" ID="BodyContent" ContentPlaceHolderID="MainContent">
    

    <table class="auto-style1">
        <tr>
            <td class="auto-style12"></td>
            <td class="auto-style11">
                            <asp:Label ID="Label7" runat="server" Font-Names="Rockwell,Consolas,Courier New,Courier,monospace;" Font-Size="22pt" ForeColor="#6e311a" style="padding: 0; margin: 0;" Text="Clearspan Login"></asp:Label>
                <br />
                            <asp:Label ID="lblErrorLogin" runat="server" Font-Bold="True" ForeColor="#CC3300" Text="Invalid Username/Password." Visible="False"></asp:Label>
                            </td>
            <td class="auto-style11"></td>
        </tr>
        <tr>
            <td class="auto-style15"></td>
            <td class="auto-style16">
                <table class="NotAssigned">
                    <tr>
                        <td class="NotAssigned">
                            <br />
                        </td>
                        <td class="auto-style7" style="text-align: center; vertical-align: middle;;padding:0px;margin:0px; ">
                            <asp:Label ID="lblUsername" runat="server" Text="Username:" Height="30px"></asp:Label>
                            &nbsp;
                            <asp:TextBox ID="txtUsername" runat="server" BackColor="White" BorderColor="#CCCCCC" BorderStyle="Inset" BorderWidth="1px" Width="160px" Height="30px"></asp:TextBox>
                        </td>
                    </tr>
                    <tr>
                        <td class="NotAssigned"></td>
                        <td class="auto-style7" style="text-align: center;padding:0px;margin:0px; ">
                            <asp:Label ID="lblPassword" runat="server" Text="Password:" Height="30px"></asp:Label>
                            &nbsp;&nbsp;
                            <asp:TextBox ID="txtPassword" runat="server" TextMode="Password" BackColor="White" BorderColor="#CCCCCC" BorderStyle="Inset" BorderWidth="1px" Width="160px" Height="30px"></asp:TextBox>
                        </td>
                    </tr>
                </table>
            </td>
            <td class="auto-style16"></td>
        </tr>
        <tr>
            <td class="auto-style13">&nbsp;</td>
            <td>
                <table class="NotAssigned">
                    <tr>
                        <td class="auto-style14" style="text-align: center;padding:0px;margin:0px; "><asp:Button ID="btnLogIn" runat="server" Text="Log in" OnClick="btnLogIn_Click" Height="30px" Width="162px" />
                        </td>
                    </tr>
                    <tr>
                        <td>
                            <asp:TextBox ID="txtHold" runat="server" Height="16px" ReadOnly="True" TextMode="Password" Width="16px" Visible="False"></asp:TextBox>
                        </td>
                    </tr>
                </table>
            </td>
            <td>&nbsp;</td>
        </tr>
    </table>
</asp:Content>
<asp:Content ID="Content1" runat="server" contentplaceholderid="HeadContent">
    <style type="text/css">
        .auto-style1 {
            width: 100%;
            height: 65px;
        }
        .auto-style7 {
            width: 633px;
        }
        .auto-style11 {
            height: 51px;
        }
        .auto-style12 {
            height: 51px;
            width: 58px;
        }
        .auto-style13 {
            width: 58px;
        }
        .auto-style14 {
            width: 712px;
        }
        .auto-style15 {
            width: 58px;
            height: 84px;
        }
        .auto-style16 {
            height: 84px;
        }
    </style>
</asp:Content>