Almost always, this happens as a result of someone using the animated sprites when their computer can't quite handle it.
If you are indeed getting this error using the animated sprites, you have a couple options:
1) If it is only a specific Pokemon that causes the hang, go into Pokemon Reborn/Graphics/Battlers find that Pokemon's animated file, and delete it. This will revert to the static sprite for that Pokemon only.
2) Revert to using the static sprites for the game over all. This is the all-inclusive fix for this problem if it happens a lot.
If you are getting an error like this while not using the animated sprites, please make a post in our Support Forum