Possibilities;
#1 most likely
1) SVN gmod/data/ULib/groups.txt AND users.txt (and I believe misc_registered.txt) uses a much different format than release. SVN has a built in converter that converts old to new.
However, release 3.4 does not have a convert new to old. If you're trying to use the converted groups.txt and users.txt files without restoring the old from backups, you'll get errors.
Look in your server's gmod/data/ULib for backups. I think SVN creates them. old<file>.txt
(Either that, or I backed my own up)
2) Mixed versions? (which, yes, technically #1 would be too). Did you totally delete/remove gmod/addons/ULib and /ULX before re-installing release?
Also make sure you've totally restarted server and if you use sv_downloadurl, have uploaded the newly rebuilt cache after restart.