Had similar problem with Sardis, as Epirus, against Seleucids (sry cbf using fancy EB spellingz)

Sieged it with superior army from the South-West tile, no AI response except moving a couple of minor stacks (1-3 units) next to my sieging army but didn't attack me, siege ended with starve and without sally. When I clicked "Enslave" or whatever the middle one is called, CTD. Reloaded QS from before clicking End Turn. AI this time attacked my stack, was attacked from the East with weak armies coming from North-West and South-East, with the city garrison from the North-East of course. Won (not without cas, hate fighting uphill) and clicked Enslave and no CTD this time.

Sounds like a different issue though