This sort of error is caused by the game loading invalid data. It could be terrain, objects or even gamplay constants / gameplay interface settings. If a custom load screen is used, the image may even be curropt. For that mater, any import being curropt has been known to do such things. Badly written triggers can cause this sort of error as well such as initializing a global quest to a quest object.
Firstly try removing all imports from the map. Then try rolling back each settings 1 by 1 until you basically get a blank unedited map that is playable.
The key to not losing the map's content is to save a backup of it in a non working state. Then break it piece by piece until it is working (and you identify what was causing the error). Then you can go back to your broken backup copy of the map and only remove or fix the piece that is broken so all other content is not lost.
