Repeatable crash. Steps to repeat:
1. Play coop campaign IE (both players on the same team).
2. AI faction demands money from Player 1.
3. Player 1 declines, causing Player 2 to automatically be prompted to join war on their side.
4. Player 2 clicks "Enter War on Side of Ally", game immediately crashes for both players.
We tested this several times (since the AI didn't always make the same threat). The crash only occurred when entering in war on side of ally after a demand for money/threat. Normal war declarations did not cause the crash to occur. The game crashed irrespective of which player was threatened. This bug did not occur before patch 2.1.
Since coop allies cannot decline the request, this nearly softlocks the campaign (technically we could keep restarting every time someone threatened us, or even accept the demand, but those aren't acceptable solutions).
For reference, we were playing Greasus and Azhag.
Dxdiag and mp_log are attached.