Mariadb tables crash constantly

What could be causing mariadb tables crashing constantly?
Debian GNU/Linux 9.7, ISPConfig 3.1.13. Apache web server.

Code:

mariadb-server-10.1:
  Installed: 10.1.37-0+deb9u1

Code:

2019-02-03 19:33:25 139971540297472 [ERROR] mysqld: Table './npertti_futsal/smf_topics' is marked as crashed and should be repaired
2019-02-03 19:33:25 139973697038080 [ERROR] mysqld: Table './npertti_futsal/smf_topics' is marked as crashed and should be repaired
2019-02-03 19:33:25 139971540297472 [Warning] Checking table:   './npertti_futsal/smf_topics'
2019-02-03 19:34:45 139973614794496 [ERROR] mysqld: Table './npertti_frisbee/smf_topics' is marked as crashed and should be repaired
2019-02-03 19:34:45 139973614794496 [Warning] Checking table:   './npertti_frisbee/smf_topics'
2019-02-03 19:34:53 139973614794496 [ERROR] mysqld: Table './npertti_frisbee/smf_members' is marked as crashed and should be repaired
2019-02-03 19:34:53 139973614794496 [Warning] Checking table:   './npertti_frisbee/smf_members'
2019-02-03 19:35:45 139972751476480 [ERROR] mysqld: Table './npertti_futsal/smf_log_online' is marked as crashed and should be repaired
2019-02-03 19:35:45 139971538691840 [ERROR] mysqld: Table './npertti_futsal/smf_log_online' is marked as crashed and should be repaired
2019-02-03 19:35:45 139971538491136 [ERROR] mysqld: Table './npertti_futsal/smf_log_online' is marked as crashed and should be repaired
2019-02-03 19:35:53 139972751476480 [Warning] Checking table:   './npertti_futsal/smf_log_online'
2019-02-03 19:35:55 139971540899584 [ERROR] mysqld: Table './npertti_wwwdeyt/nsuhi_rsseo_visitors' is marked as crashed and should be repaired
2019-02-03 19:35:55 139971773646592 [ERROR] mysqld: Table './npertti_wwwdeyt/nsuhi_rsseo_visitors' is marked as crashed and should be repaired
2019-02-03 19:35:55 139971538089728 [ERROR] mysqld: Table './npertti_wwwdeyt/nsuhi_rsseo_visitors' is marked as crashed and should be repaired
2019-02-03 19:35:55 139971774248704 [ERROR] mysqld: Table './npertti_wwwdeyt/nsuhi_rsseo_visitors' is marked as crashed and should be repaired
2019-02-03 19:35:55 139971636156160 [ERROR] mysqld: Table './npertti_wwwdeyt/nsuhi_rsseo_visitors' is marked as crashed and should be repaired
2019-02-03 19:35:55 139971540899584 [Warning] Checking table:   './npertti_wwwdeyt/nsuhi_rsseo_visitors'
2019-02-03 19:36:00 139971636557568 [ERROR] mysqld: Table './c1NuhviFoo/phpbb_topics' is marked as crashed and should be repaired
2019-02-03 19:36:00 139971539293952 [ERROR] mysqld: Table './c1NuhviFoo/phpbb_topics' is marked as crashed and should be repaired
2019-02-03 19:36:00 139971636557568 [Warning] Checking table:   './c1NuhviFoo/phpbb_topics'

Load is very high, maybe because mysql process is constantly repairing those tables.

 

This runs on a virtual host. I see no errors on other virtual hosts running on this same iron host.
I have now checked disk is not full, there is 6 GB merory free, swap in use is 70 MBytes of 2000 MBytes.
Now that I read the /var/log/mysql/error.log back, problem started today 16:52. mariadb crashed, and dumped memory map.
Maybe that crash cause several tables to crash? And now mariadb is just repairing those crashed tables? Can I force a repair of all tables in all databases?

 

The problem might have repaired itself eventually. I suspect the mariadb crash earlier today broke tables. Now it was doing repairs table by table as a broken table was used for something. Load was going down slowly, too.
I did issue command

Code:

mysqlcheck --all-databases -p

to get all fixed. Looks like that command takes hours to complete.