Tweak, though you may have tried it already, try moving/backing up all ULib data files (users.txt, groups.txt, misc_registered.txt). I know you stated you let groups.txt rebuild, but for testing try the other two also.
I was working with a clean file, I only have myself as superadmin in settings/users.txt, and it worked fine after changing myself to owner.
You're not running another admin mod are you? Though most times we see no big issue, if you are, try removing it first.
Also, and this is one I've mentioned to Megiddo... ULib now executes some files locally, even when joining a non-ULib server
We wanted to allow client side only scripts. However, even when I joined a non-ULib/ULX server once, I saw ULX files try to run. If you have ULib and ULX installed on your local client, make sure it's the same version as the server too.
I know it's a pain, but try each of the above one at a time (first ULib/ULX local update (or remove), then remove admin mod if you have more than one, then rebuild of all files, then remove all mods)
Any clue you can lead to may help us find improvement (or, worst case, know what to look for in correcting someone elses error)