Hi, thanks for sharing this issue.
This problem is occuring because postsharp is executing code that has been instrumented by NCrunch. The GAC workaround won't work in this scenario, probably because the environment used by Postsharp doesn't include the GAC in its search path.
Something you can try is to add a direct reference to the nCrunch.TestRuntime.DotNetCore.dll assembly from the project containing the instrumented code. This would cause the build system to copy the file to the build output directly where hopefully Postsharp can include it in its search path. To avoid making this a dependency for your non-NCrunch builds, you could make it conditional, for example:
<ItemGroup Condition="'$(NCrunch)' == '1'">
<Reference Include="C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\Common7\IDE\Extensions\Remco Software\NCrunch for Visual Studio 2019\nCrunch.TestRuntime.DotNetCore.dll" />
</ItemGroup>
