Kentico 11 site seems to have lost sight of mscorlib.dll

I am running a Kentico 11 basic install on Windows 10/IIS 10 for training purposes. All was well for several days, until quite suddenly Visual Studio 2022 started marking most object references in .cs files in the CMS project as errors. (These are files that not precompiled, but get compiled and cached at run time.) Hovering over the marked strings I see such messages as Predefined type 'System.Object' is not defined or imported. When I try to view any page of the site I am presented with an IIS error screen with the message Could not load file or assembly 'System.Collections' or one of its dependencies. Strong name signature could not be verified. The assembly may have been tampered with, or it was delay signed but not fully signed with the correct private key. (Exception from HRESULT: 0x80131045) Other projects in the solution seem OK. At any rate, they look OK in VS and they build without error.

I've tried changing the .NET version reference to other installed versions, with no effect. I've tried manually adding a reference to mscorlib.dll, with no effect. I've tried reinstalling .NET 4.8 (the version it was referring to to start with), with no effect. I am not aware of any change to the system that would have caused this; to my perception it was sudden and inexplicable. This system is not being used by anyone else, and I am not using it for anything but this training.

I've exhausted my limited expertise. What should I try next?