Middleearth, if you shutdown server, then move/rename/backup data/ulib groups and users.txt, and then restart server with a 'fresh' users and groups files, then add only yourself to superadmin using server console/rcon and then have someone else join, does it still happen to you?
This is the first time, in many years, I've seen this issue reported.
Other than a conflicting (with ulib or ulx) addon/gamemode feature, or a corrupt groups or users file in someway (which testing fresh should help test), it sounds like a resource issue as roastchicken suggests.