We had some issues earlier in development with ULX/ULib but I haven't spotted any issues with it lately. It's possible some other admin mods could cause issues, and mods which attempt to patch Lua or "improve performance". If you're able to identify which addons conflict please let us know and we can try to fix the conflict.

If you test some "known good" entities and find that they error, could you please paste the error here? Some entities that should work without issue include Zombie Spawner, Sonic Speedup Box, Diglett, Sentry Pill (will require TF2 mounted), Trampoline, Nyan Core, Pig. These can all be found under "Entities" "Most Favorite".