For Megiddo:
It looks like the UCLAuthed and ULibLocalPlayerReady hooks either aren't working at all, or something changed such that my functions hooked to those no longer get called. I got similar results with a listen server. Oddly enough, it seems to get called fine in single player (ignoring the Player Not Authed errors shortly after). I was able to get XGUI to start up by manually calling the correct functions in order.
For everyone else:
I just barely pushed a build that changes a few things in how XGUI starts up, but it still doesn't fully fix the problem. However, you are now able to run this command in your client console after joining a server to get XGUI working:
lua_run_cl xgui.init()
Do note that this requires your server to have sv_allowcslua set to 1. Hopefully this holds while we try and figure out what the real problem is.