I'm not quite sure what's causing this bug, so if you have any more infos on it, please send them in. Untill now we know that a variable inside the config.cfg or sdos.cfg may cause this error to happen (see below for the villains).
Here is a quick fix:
Try moving your current config.cfg and sdos.cfg to a save location and start sauerbraten.
Your config.cfg will be located in <hard drive>/Users/<your username>/library/application support/Sauerbraten/
The directory library might be hidden, so unhide it or just type it into finder and he shall find it.
Suspected villains:
config.cfg:
- /texreduce