Latest release of BattleGrounds 2 mod for Half-Life 2 (Orangebox engine). Version 2.2 - released December 19th, 2013.
The Battle Grounds 2(BG2) is a Half-Life 2 multiplayer modification based on the revolutionary war. It is the sequel to the popular HL1 mod with even more features and possibilities. The British Army and its Native American tribal allies engage in battle against the United States Continental Army and local militia forces. Interactivity is a very important factor in this mod and team play is as important as ever before. Take aim and fire muskets, prepare bayonets and take control of the battlefield to succeed in your campaign for final victory. Combat will vary from long-range rifle marksmanship to brutal close quarters hand-to-hand melee fighting. You will need a mix of first person combat skills and quick-thinking strategy to survive. So prepare yourself to do battle for your side and to meet the enemy head-on. Only one side can be victorious.
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.