It was occasional, but today I'm getting the error everywhere. I've made empty solutions, added A to B and B to A, deleted all the .vs .user and binary outputs (some searches suggest that), but absolutely nothing works. Web searches produce no modern useful results.

I'm basically dead in the water. Has anyone else had this and solved it?

I'm even running a repair now, which is pretty desperate (and slow). I know it won't work, but I do it anyway, which is a standard 21st century software development procedure.