Weird, I thought I replied to this already. I put -show_err in the command line and took Messana again, but it didn't crash. It wasn't the exact same scenario as last time because I left the island and then came back. The first time around was much messier. I had three full armies outside the city and Syracuse had 4 armies outside the city and one inside. I had to fight the battle and then take the city, and there were still piles of troops all around it. This was when the game would crash, but I ended up leaving the island and saving the game before I tried again. When I went back I just beseiged the city, clicked auto-resolve, then enslaved the population and it didn't crash.

I'm playing the campaign that uses the bigmap, which is provincial, right?

I hope the information helps. Sorry I couldn't get the crash to happen again, but I didn't think to keep the save from right before it crashed.