Thank you for pointing me at the direction. The problem occurred one more time, approx. 20 hours after the first one. Unfortunately, my colleague restarted Asterisk before I was able to collect a backtrace. Nevertheless, I figured out that the problem started occurring after enabling the TCP transport (it was not used for any line). When I disabled the TCP, the problem did not occur again (it is 8 days now). So I guess, there is some relation to the bug reported here:
https://issues.asterisk.org/jira/browse/ASTERISK-26136
The problem appeared only in our production environment, so I have limited possibilities to reenable TCP transport again. If I manage to do that and the problem appears, I will post the backtrace.
Thank you for support.