As the file states, you should never, or ever have reason, to edit /addons/ULib/data/groups.txt
ULib creates its own file based on that file. If that file is changed/corrupted, everything based on it is too.
This is most likely why ULib is reporting the error.
If any file should have been edited at all, it would be gmod/data/ULib/groups.txt, and even then we don't recommend it.
Too many times we've seen bad syntax.
Please attach your gmod/data/ULib/groups.txt and your gmod/settings/users.txt
For extra measure, go ahead and attach a "ulx debuginfo" file from the server while you and some others are connected.