Alright, since you have nothing uncommon there, it should be safe to delete both .sln and .csproj files, and see if that fixes the issue.
If it doesn't, then I guess there must be an issue with either your .NET installation (or Mono, but better not to use Mono anymore), the NuGet configuration, or some stuff within your userdata folders.
As a start, you should try and make sure that NuGet works. From the docs:
A simple way to fix the NuGet configuration file is to regenerate it. In a file explorer window, go to%AppData%\NuGet. Rename or delete theNuGet.Configfile. When you build your Godot project again, the file will be automatically created with default values.
I can recall this fixing the issue on one of my machines once. I really hope we won't have to reinstall stuff, I wouldn't want you to go through that trouble.