FIXED: i had the same issue. I went to C:\Users\USERNAME\AppData\Local\unityhub-updater and ran the installer from there. That reinstalled the unity hub and everything worked normally. All my projects were there as well. You may have to sign in again tho. hope this helps someone!

Deleting the UserSettings/Layouts/default-2021.dwlt file resetted my window layout, so I guess that makes sense. And also all my Testrunner results and everything..
I restarted Unity and it seemed to recompile the assemblies on startup, but whenever I changed some code, the "Reload Script Assemblies of death" came back compiling for like 7 Minutes.

After all: It did not solve my problem.. Still Reload Script Assemblies is busy forever.

Don't know if this will be of use to anyone, but when I was getting frozen on reload script assemblies (infinitely frozen), I did a LOT of guess-and-checking, and found that deleting the UserSettings/Layouts/default-2021.dwlt in my project's folder solved the problem. I have no idea what that file is for or if I'm doing something bad by deleting it, but it works. If anyone can tell me what that file is for, I'd be grateful.

This can also happen when there's an invalid link junction (one that points to a non-existing folder).
It also comes with heavy disk usage, which is strange... I left it open for hours while waiting for the possibility it may resolve by itself.