Unfortunately, it's a glitch assosciated with the core engine thus you can't do anything to fix or avoid it. It's triggered by a lack of adequate free memory.
I'll try my best to explain how and why this particular error happens: (anims are animation files that each ai uses to make their model move and behave in the correct way)
At any given time during play, there can only be 24 (or 48?) AI zombies in your game. When one dies, it is almost immediately replaced by the game spawing in another one close to a player. (If you go to theater and fix the camera on 1 zombie, after it dies, you'll notice your camera will suddenly 'jump' after a few seconds to a new random zombie that just spawned in the map).
So in short; there are 24 total anims reserved for exactly 24 zombies.
So say you're training on round 60. At this round, the amount of free memory on consoles is slowly starting to become a problem for the game.
When you turn and kill let's say 6 zombies in a row, the game can glitch out because of memory problems and actually spawn in 7 zombies instead of 6.
Meaning you'll have 25 ai zombies in your game that all require anims but still the game only has 24 anim files available.
Leaving one zombie without the necessary animation files.
And thus your game will stop running to prevent further issues.
It sucks.