Hi Tamas,
sorry for the very long delay in answering.
Unfortunately this report doesn't contain enough information to work on:
there is no log, no backtrace, no indication of what exactly crashes
(the cinnamon process restarting itself, or the whole session going back
to the display manager), and no way to reproduce it. Repeated Cinnamon
crashes are most often caused by a third party applet or extension, but
without any data that can only be a guess.
The versions involved, cinnamon 3.2.7-4 on Debian 9 stretch, have also
been out of support for years, and current Cinnamon is six major series
ahead of that.
I'm therefore closing this bug. If you still see the problem on trixie or
sid, please open a new report including the output of
journalctl --user -b | grep -i cinnamon
and tell us whether it still happens with all third party applets,
desklets and extensions disabled.
Thanks for the report,
Fabio