I gathered another gdm log file featuring a full Backtrace and Memory Map. The problem happens also with version 1:12-2-2 to which I reverted and have been running for several weeks. So I don't know where the culprit is, maybe some other package update (xorg?) or some other thing I may have done this morning but I cannot figure out.
Thanks, Carlo.
:61.log.2
Description: Binary data