[ERROR] addons/ulib/lua/ulib/server/ucl.lua:1043: attempt to call upvalue 'playerAuth' (a nil value)
So it is not third-party code, because that would consistently throw this error every time a player connects.
So it is not ULib, because ULib does not change playerAuth during a game session.
It happens after a player connects to the server usually after nobody is on the server for a long period of time.
I wonder if this has something to do with how Garry's Mod servers hibernate.
I can reproduce this error by forcing an autorefresh for ucl.lua (though not on the same line as you).
Maybe something is causing ucl.lua to refresh. The code in ucl.lua is not supposed to run more than once.
Can you try the following:
Disable hibernationAdd the following line to your server.cfg file:
sv_hibernate_think 1
Disable autorefreshAdd the following option to your server startup command:
-disableluarefresh
Restart your server to apply these changes.
Once this happens...the server gets laggy...
An uptime of 6+ hours will (also) cause jitter. Source: see "Note" on
https://wiki.facepunch.com/gmod/Global.CurTime.