http://bugs.winehq.org/show_bug.cgi?id=13560
--- Comment #4 from Christian Apolloni christian.apolloni@gmail.com 2008-08-05 12:33:11 --- 1. install BG2:SOA 2. install BG2:TOB 3. start a new game (or load a saved game) 4. try to save -> error message
If you manually remove the default.tot file just before trying to save, the save is successful.
This default.tot file gets recreated pretty frequently so removing it does not solve the problem once for all. My guess is that it gets created every time you load or save a game.