Valve updated something in Steam, causing Source 2007 mods to break. The symptom caused is a Steam ID Ticket error. This is a Valve-side error, not something unique to Battle Grounds 2.
A possible temp fix is here:
NOTE: The error does not seem to be client-side, but rather server side. Hopefully Valve will fix this error.
Feel free to voice your concern here:
Errors like these have the power to put mods out of commission. Let Valve know you would like to see this error fixed.