I’ve personally never run across that problem. I did a bit of searching on the forum.
I found these two article that might help:
Windows: Errors trying to upgrade, install, or uninstall Plex Media Server
Also, this thread: Unable to Update to the newest version. It mentions some things you’ve already tried. It also mentions stopping the Plex Updater service first.
If you are able to run the uninstaller, and want to fully remove Plex from your system (to start over from scratch, etc), see the Uninstall Plex Media Server support document. It mentions a registry location to remove and also the %LOCALAPPDATA%\Plex Media Server folder.
