Visual Studio 2019 build errors

I have API project (.NET Framework 4.6) that contains a lot of different controllers, nuget packages, swagger, EF etc.
If i load this project & make a build – everything succeeded (No issue found) and i can deploy my project to DevOps. However if i add atliest one line of code & make a build i get more that 6000 errors.
I already tried to clean & rebuild my project but this does not help.