I found an error in cron log...

admin/cli/adhoc_task.php --showsql --showdebugging --execute

Thanks to the log via the above command, I found out that in one table of the database (within the Reservation plugin) there is a record for a user that no longer exists.
The reservation request persisted in the given reservation from this user (table reservation_request). Therefore, I believe that the problem has arisen (see log, reservation with ID 88). Now cron works without error...