If your MySQL or MariaDB server is rebooting itself using something like ISPConfig:

max_connections=50
innodb_buffer_pool_size = 512M

Other settings is custom:

innodb_file_per_table = 1
innodb_buffer_pool_size = 512M
max_connections=50
max_allowed_packet=256M
table_cache=512
sort_buffer_size=4M
myisam_sort_buffer_size=64M
thread_cache=128
wait_timeout=300
thread_concurrency=1

Error:

161213 11:49:56 mysqld_safe Number of processes running now: 0
161213 11:49:56 mysqld_safe mysqld restarted
161213 11:50:00 [Note] /usr/sbin/mysqld (mysqld 10.0.28-MariaDB) starting as process 1233 …
161213 11:50:00 [Note] InnoDB: Using mutexes to ref count buffer pool pages