UE5/Rider - tons of incorrect error messages on build

I have a working UE5 project that compiled perfectly until yesterday afternoon. Suddenly any attempt to build reports errors on almost every line of code - clearly things that are not actually errors. A handful of these are flagged in red by Rider, but most are only listed when trying to build. They seem to largely involve missing symbols (or errors cascading from that) - almost like it's failing to properly include files that are definitely included.

This began when I added one single line of logging to a random class method. Commenting that line out changes nothing and the code compiled/ran perfectly before that.

I've rebuilt the solution, cleared the cache, restarted my machine, done every combination of all 3 in every order I can think - but the same issues still persist. I upgraded Rider, but nothing changed. All I can think is that something corrupted a cache somewhere that I'm not aware of and don't know how to clear.

Has this happened to anyone else? What more can I try to fix this?