error ASPCONFIG: Unable to open configSource file - asp.net-mvc

Question

Our .Net Framework 4.8 web application suddenly began to generate the following build-time error on our team city build server:
error ASPCONFIG: Unable to open configSource file ‘sessionState.config’.
Our web.config references an external session state file:
<sessionState configSource="sessionState.config" />
The project is under active development, however no related changes have been recently made to the application. We initially thought it was a configuration issue on the build server, however we realised that we could replicate the same issue on our development machines if we "Published" the project and ticked the option to "merge all outputs into a single assembly". We published to a local file to replicate what was happening on our build server.
Modifying the Build Action of the sessionState.config file from None to Content removed the problem, but this had the effect of copying our sessionState.config file to the deployment package, which we didn't want.

Answer 1

We spent many hours trying to investigate this problem and finally decided to try uninstalling recent Microsoft Updates. This resolved it! I'm posting this here in the hope that it will save at least one other person the time we spent.
This specific October 13, 2020 update was the cause, however, it didn't come up for us when we searched https://support.microsoft.com/en-us/help/4578974/kb4578974-cumulative-update-for-net-framework
After you apply this October 13,
2020 Security and Quality Rollup for .NET Framework 4.8, some ASP.Net
applications fail during precompilation. The error message that you
receive will likely contain the words “Error ASPCONFIG.”
An invalid configuration state in either the "sessionState,"
"anonymouseIdentification," or "authentication/forms" sections of
"System.web" configuration. This might occur during build-and-publish
routines if configuration transformations leave the Web.config file in
an intermediate state for precompilation.
Note their spelling error in anonymousIdentification
The solution is to add this appSettings key to your web.config
<configuration>
<appSettings>
<add key="aspnet:DisableAppPathModifier" value="true" />
</appSettings>
</configuration>
The article notes that setting the value to either "true" or "false" will avoid the issue. It was recommended to set this value to "true" for sites that use cookies for the session state id.
We use cookies, so added this to our web.config with a true value, and it resolved the problem.