We've seen this error pop up a few times over the past three years.
I thought it had been wiped out.
Here's some things I remember that various folks have done to fix.
1) Make sure your using OUR svn link (ulyssesmod.net) - I've seen some hosting providers offer SVN of our releases have OLD code.
2) Try removing ULX and ULib addon folders from your local machine.
3) Try removing ALL local client addons if #2 doesn't work.
4) Use PAC (player accessory something addon, hats, etc), any version, anywhere? We've seen it cause lots of trouble (not sure it was this error though)
5) Try clearing out your local lua cache folder ; usually c:\<steam path>\garrysmod\cache\<delete all in here> <--- joining servers will again take a while, but if caches have become corrupt with different versions of ULib/ULX, could be causing issue
6) Use FastDL? Tried clearing out/reuploading your cache lately.
Last but not least, and other ideas may spring up... after #3, go ahead and try removing all server addons but ULib ULX.
It's a pain, I know, but after removing all, restarting server, reuploading cache if fast dl...hopefully will determine its a conflict not in our code.
7) Are you playing on a legitimate copy over internet? Or using some private lan tool that tries to act as internet?
ALSO, if you know how, search (using your tool of choice) inside all your lua files for "UniqueID" - all besides ULX and ULib.
I'm betting another mod is overwriting it. Our code iuses ply:UniqueID, and other than non-legit or non-internet games, would probably be another bit of code wiping it out (including Gmod itself, maybe).