Same here, first i thought the patch had broken a mod, so I tried disabling them all. Then i wondered if it was a safefile error, so i reloaded back a few minutes and went through the siege, it then crashed again as I tried to enter. Then had a hunch and waited for the vote for ownership, and could enter just fine.
To be more specific, it only seems to crash after a siege, before the vote for the new owner has concluded, seems to work fine "post-vote". Only tested with castles so far, not sure about cities.
*edit: Not sure if relevant, but during one of the "trial reloads" to figure it out, i saw the "siege icon" (catapult) over the castle despite the siege being over (me taking the castle).