Not enough memory?
Feb 4 01:10:42 kernel: [10323.321102] Out of memory: Kill process 6700 (mysqld) score 170 or sacrifice child
Feb 4 01:10:42 kernel: [10323.338759] Killed process 6700 (mysqld) total-vm:646748kB, anon-rss:85280kB, file-rss:0kB, shmem-rss:0kB
Feb 4 01:10:42 systemd[1]: mariadb.service: Main process exited, code=killed, status=9/KILL
Feb 4 01:10:42 systemd[1]: mariadb.service: Unit entered failed state.
Feb 4 01:10:42 systemd[1]: mariadb.service: Failed with result 'signal'.
Feb 4 01:10:47 systemd[1]: mariadb.service: Service hold-off time over, scheduling restart.
Feb 4 01:10:47 systemd[1]: Stopped MariaDB 10.1.37 database server.
Feb 4 01:10:47 systemd[1]: Starting MariaDB 10.1.37 database server...
Feb 4 01:10:47 mysqld[7045]: 2019-02-04 1:10:47 140575933501888 [Note] /usr/sbin/mysqld (mysqld 10.1.37-MariaDB-0+deb9u1) starting as process 7045 ...
Feb 4 01:10:48 systemd[1]: mariadb.service: Main process exited, code=exited, status=1/FAILURE
Feb 4 01:10:48 systemd[1]: Failed to start MariaDB 10.1.37 database server.
Feb 4 01:10:48 systemd[1]: mariadb.service: Unit entered failed state.
Feb 4 01:10:48 systemd[1]: mariadb.service: Failed with result 'exit-code'.
Feb 4 01:10:49 froxlor[7049]: SQLSTATE[HY000] [2002] Connection refused
Feb 4 01:10:49 froxlor[7049]: --- DEBUG: #0 /var/www/froxlor/lib/classes/database/class.Database.php(297): PDO->__construct('mysql:dbname=st...', 'stuser', 'DB_UNPRIV_PWD...', Array) #1 /var/www/froxlor/lib/classes/dat$