Ok, so I used stress-ng to stress test ram for 6 hours, no problem so far.
Did a copy of /var/lib/mysql just in case.
I did a scrub on the filesystem, no error, so reset the btrfs corruption counter.
I restarted mariadb with innodb_force_recovery=5 as suggested by mariadb, and exported all possible databases. Then dropped the corrupted databases.
Removed files causing problems in /var/lib/mysql, restarted mariadb removing innodb_force_recovery=5 first and recreated mysql needed tables with mysql_upgrade -u root -p --force, and reimported databases I wanted, and then imported nextcloud app backup.
Everything is working for now, don’t have errors any more.
Will have a close look at it for the next weeks to see if everything still running fine 
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