I'm jealous of all of you who resolved through File / Invalidate caches. I just spent hours trying everything on this question and a few others from around the web.
The magic menu item didn't do it for me so I invalidated the caches myself by nuking %USERPROFILE%\AppData\Local\JetBrains\IntelliJIdea2020.3\caches
Once I did this and restarted IntelliJ (2020.3), the indexes were rebuilt and my errors went away.