Why restarting mariadb is causing service to fail with this error

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mariadb.service - MariaDB 10.1.47 database server
Loaded: loaded (/lib/systemd/system/mariadb.service; enabled; vendor preset: enabled)
Active: failed (Result: exit-code) since Fri 2020-11-27 23:50:06 GMT; 33s ago
Docs: man:mysqld(8)
https://mariadb.com/kb/en/library/systemd/
Process: 3067 ExecStart=/usr/sbin/mysqld $MYSQLD_OPTS $_WSREP_NEW_CLUSTER $_WSREP_START_POSITION (code=exited, status=1/FAILURE)
Process: 2921 ExecStartPre=/bin/sh -c [ ! -e /usr/bin/galera_recovery ] && VAR= || VAR=cd /usr/bin/..; /usr/bin/galera_recovery; [ $? -eq 0
Process: 2919 ExecStartPre=/bin/sh -c systemctl unset-environment _WSREP_START_POSITION (code=exited, status=0/SUCCESS)
Process: 2917 ExecStartPre=/usr/bin/install -m 755 -o mysql -g root -d /var/run/mysqld (code=exited, status=0/SUCCESS)
Main PID: 3067 (code=exited, status=1/FAILURE)
Status: “MariaDB server is down”
CPU: 1.023s

Nov 27 23:49:28 rockro systemd[1]: Starting MariaDB 10.1.47 database server…
Nov 27 23:49:32 rockpro sh[2921]: WSREP: Recovered position 00000000-0000-0000-0000-000000000000:-1
Nov 27 23:49:32 rockpro mysqld[3067]: 2020-11-27 23:49:32 547567972368 [Note] /usr/sbin/mysqld (mysqld 10.1.47-MariaDB-0ubuntu0.18.04.1) starting
Nov 27 23:50:06 rockpro systemd[1]: mariadb.service: Main process exited, code=exited, status=1/FAILURE
Nov 27 23:50:06 rockpro systemd[1]: mariadb.service: Failed with result ‘exit-code’.
Nov 27 23:50:06 rockpro systemd[1]: Failed to start MariaDB 10.1.47 database server.
Nov 27 23:50:06 rockpro systemd[1]: mariadb.service: Consumed 1.023s CPU time

Nextcloud version (eg, 18.0.2): 20.1.1
Operating system and version (eg, Ubuntu 20.04): 18.04
Apache or nginx version (eg, Apache 2.4.25): 2.4
PHP version (eg, 7.1): 7.4

The issue you are facing:
mariadb service not starting.

Due to the fact that this is a general database and not a Nextcloud specific error you should ask aunt G. for help to get this problem solved: