Comment 36 for bug 588155

After not having seen this bug for a while, it finally appeared again and I got the backtrace (see attachment). I had the breakpoints set on exit and _exit as suggested by David. I hope this helps and someone can take a look at this soon. BTW the .xsession-errors showed the following error:

The program 'gnome-settings-daemon' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadMatch (invalid parameter attributes)'.
  (Details: serial 1355 error_code 8 request_code 3 minor_code 0)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the --sync command line
   option to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)

Greets, Martin