Many people including myself have played both these maps in co-op to high rounds, and many other maps, what exactly happens?

We would be playing and I would get a crash saying that the AI could not ...

For custom zombies maps such as Nacht Der Untoten Remastered and Das Herrandas (All custom zombie maps through UGX) throughout while playing around middle of the game ( around round 6) the game would eventually crash we didnt have this problem for regular maps that are on the actual game...Please He...