I don't know for sure, but in some cases it's one of the custom meshes that you might imported from somewhere that cause things like these, I spose anything custom in a map really.
If you know that the exporting actually works with any map(not failing to export every single time no matter which map).
I'd suggest you trying to remove all the custom things inside the map and then try to re-export it, could be billboards, meshes, particles, anything that isn't from the game itself.
I don't know for certain, but that's what I'd try. Sorry if I don't have anything better to say right now.