I'm not sure. Had twice a CTD when attacking the Seleucids for the same battle. Unfortunately I overwrite the save game of this battle. When I start a custom battle with the same factions (Far East Celts and Seleucids) this also starts a CTD but after this CTD I couldn't replicated it anymore. Looking in the units of the Seleucids I noticed the amount of pikemen is 123 instead of 121 when you playing with large army. The reason is they have three man carrying a flag.