I deleted all those .plgs before rebooting my server. That apparently solved my issue. Well, most likely the reboot solved the issue. Also, with a clean shutdown I was able to resume the preclear process without any problem.
Couple of days went by, yesterday I finally upgraded my parity drive so I'm now in the middle of a parity rebuild. At least I think so, because the same problem just came up again. But now I'm not going to risk a reboot before the rebuild process is over, so I have to wait a couple days before regaining access to my WebUI.
Do you have any other suggestion? Does anyone happen to have the same problem?