You didn't specifically state what kind of crash do you get, did you?
acknex has stopped working, script crash in ..., ? Have you used sys_marker already?

It could just be an action (a function) from a previous level that continues to run after the level_load, which then deals with an invalid pointer (not necessarily an entity). The error would be to look for in the level from which you switch to the seemingly crashing level.
I was recently experiencing similar problems with entity functions not being stopped on level_change. Do you have let's say a rocket that shoots high into the sky or some random bird flying high? If they leave the level(_ent)/ ABT boundaries this could lead to problems later on.