We have a suite of 10,000 unit tests which ran with no issues prior to the 2018.3 update. After the update, I have 45 failures with these Mixed mode assembly errors mostly. I googled this error and found that similar (non-reshaper) issues of this type require adding
<startup useLegacyV2RuntimeActivationPolicy="true"> </startup>
to app.config files. I created an app.config file for unit test dll project causing the majority of these errors and also verified that C:\Program Files (x86)\Microsoft Visual Studio 14.0\Common7\IDE\CommonExtensions\Microsoft\TestWindow\x64\TE.ProcessHost.Managed.exe.config had this setting in place. None of these changes has made any difference in outcome.
The issues seem to stem from a 3rd party hardware dll that we reference that uses .net 2.5. I can run Microsoft's Test Explorer with none of these issues.
What can we do to eliminate this problem?