It's been a long time since I've heard this reported in anything but DarkRP.
The last time I did see this issue and it not be DarkRP, a person was using an autopromote script, and had hundreds (maybe thousands?) of users 'promoted' to a non-default group.
(But, the were using MUCH lower autopromote time.. 1 or 2 hours I think, which, isn't really hard to do so the list grew too much)
I'd make guesses, which might be wrong, that ULib, ULX, Autopromote, and a huge user list are getting out of sync when a player joins, and one ends up winning.
Difficult to determine which one.
Add to that a heavy load on the server to Gmod game/shared host/whatever, or a crash, and any one of the scripts has more chance of being confused.
It sounds as though Autopromote is running first/faster, if donators are getting placed into stammspieler before ULib determines/Autopromote detects that players are actually donator.
Example - donator joins, Autopromote sees time and group as "user", changes access to stammspieler, THEN ULib initializes and sees actual group update different than what it originally had in file,
Summarized, I'd blame Autopromote. Not in a malicious "it's bad" way, just a "it seems to be changing access faster than ULib can tell it not to" way.
Again, just guessing from what it sounds like.
Any errors in server console (or clients?) during server startup or player connect when any of these issues occur?
