Logging out took a few dozens of seconds. The journalctl output shows
issues with nouveau and Xorg at that time:

Jul 20 17:39:39 qaa kernel: nouveau 0000:01:00.0: Xorg[1491]: failed to idle channel 4 [Xorg[1491]]
Jul 20 17:39:40 qaa kernel: audit: type=1400 audit(1784561980.053:3137): apparmor="ALLOWED" operation="open" class="file" profile="Xorg" name="/etc/glvnd/egl_vendor.d/" pid=1042547 comm="Xorg" requested_mask="r" denied_mask="r" fsuid=0 ouid=0
Jul 20 17:39:50 qaa kernel: nouveau 0000:01:00.0: Xorg[1042547]: job timeout, channel 4 killed!

and

Jul 20 17:40:02 qaa kernel: nouveau 0000:01:00.0: gsp: rc engn:00000001 chid:4 gfid:0 level:2 type:109 scope:1 part:233 fault_addr:0000000000000000 fault_type:00000000
Jul 20 17:40:02 qaa kernel: nouveau 0000:01:00.0: fifo:c00000:0004:0004:[Xorg[1042547]] errored - disabling channel

Hi Vincent,

From the report it is not clear to me if that is a regression you
experience from a previous kernel.

if so, and you have the latest working version and can reliably
trigger the problem, then it would be great if you can bisect the
problem (let me know if you need instructions).

First I would though suggest to as well test the most recent 7.1.5
kernel in unstable to verify if the problem persists.

Regards,
Salvatore