I feel guilty asking a question like this around here, but I\'m at a loss and would appreciate some help.
A proof-of-concept like web application was built on one PC
I have tried all solutions described before, but none worked.
What solved it for me, was to update the Microsoft.Net.Compilers from the NuGet Package Manager
My project was built with .Net Core 2.2 but I had .Net core 3.0 preview installed. I uninstalled .Net Core 3.0 from my system and went through all my class libraries, removing Microsoft.Net.Compilers 3.0, then rebuilt and it worked.
Deleting the bin folder worked for me