Make sure your server has write access to it's /data/Ulib and /data/ulx folders.
ULX groups (Ulib actually, though controlled through ULX) are stored in /data/ULib
I could potentially see, if all you did was a copy from old drive and restore to new, the permissions getting set as write by (what it thinks is) old user only.
There may also be some addon conflict, or the gamemode itself. I know you say you didn't add anything, but sometimes, the order you do things/add things in Gmod on an old system, for whatever reason, isn't always the same when you copy exact same stuff to a new.
If you try just Sandbox, ULib, and ULX, no other addons, add a user to some group...restart server (with no crash), does it work then?