Hello Severin,
Thank you for information. It seems that our logs are too big...
We are facing this issue: https://tracker.moodle.org/browse/MDL-56223
If I go to random old course with assignments, with no students, and go to Report > Logs, I see that "Calendar event updated" is running everyday, multiple times per day, so it generates ton of logs and stores in mdl_logstore_standard_log table.
Would it be safe to run sql query in phpmyadmin and delete "Calendar event update" records from mdl_logstore_standard_log table?
This issue is still unresolved. I will try to post about this issue in forums, maybe someone faced this issue and has a solution.
Marius.