You tried "/importcube mapname"? the map has to be in the "cube" folder, so not in the base folder!
Maybe that helped...
P.S.
I've done it once some years ago, and it wasn't failing those days...
Edit:
Tried again and came to this conclusion:
- Maps from Assaultcube version 1.0 still get imported.
- Maps from Assaultcube version 1.03 and 1.04 won't get imported, the engine just crashes (in sauerbraten linux-version).
Probably there hasn't been often communicated trough updates by both games :S?
Anyways, there's your answer:
Load maps from Assaultcube 1.0 or any earlier version...