I'd first start by process of elimination.
With the recent Gmod updates, and all the addons you use, you're just asking for conflicts if one doesn't work right.
Shutdown server.
Remove all but ULib/ULX latest SVN.
Restart server.
In server console, look for errors related to ULib and ULX at server startup, and when players connect.
Join server.
Look for errors in your local console when you connect, or try to run commands.
Let us know what/if any, and how you replicate them.
If you have errors, run, from your server console, a "ulx debuginfo" and attach the file it tells you it output to your next post
If no errors, add each addon back, one big one at a time, restarting server every time.
I'd say your chances of finding out what is having problems isn't really ULX are about 80% (towards not.)
I'd say higher, but, Gmod was updated lately, and it seems we always have to find new fixes for Garry's 'updates'.