This issue is always caused by the PlayerAuth hook getting aborted before ULX has a chance to use it.
This could be caused by a poorly coded addon (check for other lua errors in your startup)
I've also seen where a user had improperly setup his users.txt file in garrysmod/settings/ and that caused a problem with his loading as well.
Since I don't have access to your server, you'll need to troubleshoot this yourself.
Here are my suggested steps:
1. Remove ALL addons from your addons folder. This includes unsubscribing from workshop addons.
2. Also remove ULX/ULib from your addons folder.
3. Remove the ULX and ULib folders from garrysmod/data
4. Make sure your garrysmod/settings/users.txt file is there and that IF you have modified it, it is properly modified. (you can post the contents here if you are unsure).
5. Download the latest ULib/ULX from github and install those ONLY.
6. Start up your server in SANDBOX gamemode with ONLY ULX and ULib installed.
7. See if you get that error.
IF you get the error STILL after following ALL of my steps above, post here and we can look into it. PLEASE make sure you have the latest versions from our github.
If you do NOT get the error and things seem to be working, follow the next steps until the problem comes back or doesn't... (I've seen weirder things) ONLY GO TO THE NEXT STEP IF THERE ARE NO ERRORS. What I'm saying is you need to start up your server after each step to check.
1. Change the gamemode of your server back to whatever it was. (DarkRP, TTT, Deathrun, w/e)
2. Start adding addons back. I suggest doing this 2 or 3 at a time so that if your server does break you'll have a better idea of what is doing it and can more easily narrow it down.
I know it's a lot of work, but I assure you. The current version of ULX and ULib work just fine. I have used it recently and it works great. No errors. This means, unfortunately, that any errors you are getting are being caused by something that is breaking it, which is outside of our control.