Unfortunately I cannot attach gdb to the X process due to gdb bug #422007. Downgrading gdb to the testing version didn't succeed either, probably because that version has other bugs on x86_64... So I won't be able to get a stack trace unless someone knows a workaround. (BTW I have seen in the BTS that other people have run into the same problem when trying to get a stack trace, e.g., #419986.) Marcus