But I was a bit loathed to delete my whole App_Data\Temp directory, and upon noticing
that the Blink sample still compiled, I just saved my non-compiling sketch, with Save-As and gave it a new name - and that too allowed me to compile again.
The solution (post #2) in the linked topic referred to a specific directory (C:\Users\\AppData\Local\Temp).
That directory has changed (not sure in which IDE version) to C:\Users\\AppData\Local\arduino and deleting that directory should be sufficient for the given problem.