I have noticed multiple times since version 3.5 that if an AI attacks another AI civilization with ~15 units in a city and that civ only has 1 or 2 cities left, the AI just stand in front of the city and waits. It seems to be a deadlock, where the attacking AI waits for a better opportunity, but is not strong enough to do something. I had the situation where the Russians only had one city with 20 units and 4 AIs wanted to attack it at the same time, but all of them just stood there doing nothing.

I resolve the problem by either going there bombarding the town and attacking it, or by forcing the AI to make peace. First then, the deadlock is broken.

A possible solution could be some timeout, where the AI cancels the attack or just throw in its army somewhere to being able to continue the game.

I play RI 3.5 hotfix 4 on price and monarch level.

The problem disappear by itself after time, when new troops arrive or through some other external intervention. However it is not a desirable behaviour and if there would be something to tweek, it would be good.