This may be (! Some savegames may be unable to ascend to other floors. The issue is being investigated) but I'm not sure, maybe another save file will help. It's happening very consistently, I'm on level 3 and whenever I try changing level, it segfaults. Either going up or down one level or going to the pocket dimension, it segfaults. I've got all the official expansions installed and in use.
I am having a similar problem. When I try to load a saved game created before the patch or after I finish a section of the tutorial the game segfaults. This is on Lubuntu 12.04 64-bit with the 3.5.0-6 kernel. Link to save file plus core dump: http://bayfiles.com/file/j4RD/E2ZEHs/load_save_crash.zip