Warning: vgui.Create failed to create the VGUI component (DPanel)
Tried to create panel with invalid base 'DPanel'
This is your problem right here- It appears that XGUI attempted to create a DPropertySheet (which is a child of DPanel), and it failed. Since we don't modify the derma/VGUI creation code past that point, I can only say that a different addon or gamemode is causing this problem. Does it only occur when certain players join? Try seeing if it works on sandbox, or remove addons until it starts working, then let us know what you found.