The server periodically issues a 500 error + MariaDB restarts every 4 minutes - any ideas?

Question

linux

Save Help))) I have no idea what to do and how to find the error...
Apache 2.4.6-90.el7.centos
MySQL 5.5.64-1.el7
PHP 5.4.16-46.1.el7_7
Perl 5.16.3-294. el7_6
Kernel 3.10.0-1062.9.1.el7.x86_64

log messages
Oct 31 10:04:07 new mariadb-prepare-db-dir: Database MariaDB is probably initialized in /var/lib/mysql already, nothing is done.
Oct 31 10:04:07 new mariadb-prepare-db-dir: If this is not the case, make sure the /var/lib/mysql is empty before running mariadb-prepare-db-dir.
Oct 31 10:04:07 new mysqld_safe: 201031 10:04:07 mysqld_safe Logging to '/var/log/mariadb/mariadb.log'.
Oct 31 10:04:07 new mysqld_safe: 201031 10:04:07 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql
Oct 31 10:04:10 new systemd: Started MariaDB database server.
Oct 31 10:05:01 new systemd: Started Session 43549 of user root.
Oct 31 10:05:01 new systemd: Started Session 43546 of user root.
Oct 31 10:05:01 new systemd: Started Session 43548 of user root.
Oct 31 10:05:01 new systemd: Started Session 43547 of user root.
Oct 31 10:05:01 new systemd: Started Session 43550 of user root.
Oct 31 10:05:01 new systemd: Started Session 43551 of user root.
Oct 31 10:05:01 new systemd: Started Session 43552 of user root.
Oct 31 10:05:01 new systemd: Started Session 43553 of user root.
Oct 31 10:06:01 new systemd: Started Session 43555 of user root.
Oct 31 10:06:01 new systemd: Started Session 43554 of user root.
Oct 31 10:06:01 new systemd: Started Session 43556 of user root.
Oct 31 10:07:01 new systemd: Started Session 43557 of user root.
Oct 31 10:07:01 new systemd: Started Session 43559 of user root.
Oct 31 10:07:01 new systemd: Started Session 43558 of user root.
Oct 31 10:08:01 new systemd: Started Session 43560 of user root.
Oct 31 10:08:01 new systemd: Started Session 43561 of user root.
Oct 31 10:08:01 new systemd: Started Session 43562 of user root.
Oct 31 10:08:01 new systemd: Started Session 43563 of user root.
Oct 31 10:08:02 new systemd: Stopping MariaDB database server...
Oct 31 10:08:04 new systemd: Stopped MariaDB database server.
Oct 31 10:08:04 new systemd: Starting MariaDB database server...
Oct 31 10:08:04 new mariadb-prepare-db-dir: Database MariaDB is probably initialized in /var/lib/mysql already, nothing is done.
Oct 31 10:08:04 new mariadb-prepare-db-dir: If this is not the case, make sure the /var/lib/mysql is empty before running mariadb-prepare-db-dir.
Oct 31 10:08:04 new mysqld_safe: 201031 10:08:04 mysqld_safe Logging to '/var/log/mariadb/mariadb.log'.
Oct 31 10:08:04 new mysqld_safe: 201031 10:08:04 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql
Oct 31 10:08:07 new systemd: Started MariaDB database server.
Oct 31 10:09:01 new systemd: Started Session 43565 of user root.
Oct 31 10:09:01 new systemd: Started Session 43564 of user root.
Oct 31 10:09:01 new systemd: Started Session 43566 of user root.

Maria's log:

201031 10:16:10 Percona XtraDB ( www.percona.com ) 5.5.61-MariaDB-38.13 started; log sequence number 1966363809
201031 10:16:10 [Note] Plugin 'FEEDBACK' is disabled.
201031 10:16:10 [Note] Server socket created on IP: '127.0.0.1'.
201031 10:16:10 [Note] Event Scheduler: Loaded 0 events
201031 10:16:10 [Note] /usr/libexec/mysqld: ready for connections.
Version: '5.5.64-MariaDB' socket: '/var/lib/mysql/mysql.sock' port: 3306 MariaDB Server
201031 10:20:02 [Note] /usr/libexec/mysqld: Normal shutdown
201031 10:20 :02 [Note] Event Scheduler: Purging the queue. 0 events
201031 10:20:02 InnoDB: Starting shutdown...
201031 10:20:04 InnoDB: Shutdown completed; log sequence number 1966363819
201031 10:20:04 [Note] /usr/libexec/mysqld: Shutdown complete
201031 10:20:04 mysqld_safe mysqld from pid file /var/run/mariadb/mariadb.pid ended
201031 10:20:05 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql
201031 10:20:05 [Note] /usr/libexec/mysqld (mysqld 5.5.64-MariaDB) starting as process 14360 ...
201031 10:20:05 InnoDB : The InnoDB memory heap is disabled
201031 10:20:05 InnoDB: Mutexes and rw_locks use GCC atomic builtins
201031 10:20:05 InnoDB: Compressed tables use zlib 1.2.7
201031 10:20:05 InnoDB: Using Linux native AIO
201031 10:20:05 InnoDB: Initializing buffer pool, size = 8.0G
201031 10:20:05 InnoDB: Completed initialization of buffer pool
201031 10:20:05 InnoDB: highest supported file format is Barracuda.
201031 10:20:05 InnoDB: Waiting for the background threads to start
201031 10:20:06 Percona XtraDB ( www.percona.com ) 5.5.61-MariaDB-38.13 started; log sequence number 1966363819
201031 10:20:06 [Note] Plugin 'FEEDBACK' is disabled.
201031 10:20:06 [Note] Server socket created on IP: '127.0.0.1'.
201031 10:20:06 [Note] Event Scheduler: Loaded 0 events
201031 10:20:06 [Note] /usr/libexec/mysqld: ready for connections.
Version: '5.5.64-MariaDB' socket: '/var/lib/mysql/mysql.sock' port: 3306 MariaDB Server
201031 10:24:01 [Note] /usr/libexec/mysqld:
201031 10:24:01 [Note] Event Scheduler: Purging the queue. 0 events
201031 10:24:01 InnoDB: Starting shutdown...
201031 10:24:06 InnoDB: Shutdown completed; log sequence number 1966363829
201031 10:24:06 [Note] /usr/libexec/mysqld: Shutdown complete
201031 10:24:06 mysqld_safe mysqld from pid file /var/run/mariadb/mariadb.pid ended
201031 10:24:06 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql
201031 10:24:06 [Note] /usr/libexec/mysqld (mysqld 5.5.64-MariaDB) starting as process 15334 ...
201031 10:24:06 : The InnoDB memory heap is disabled
201031 10:24:06 InnoDB: Mutexes and rw_locks use GCC atomic builtins
201031 10:24:06 InnoDB: Compressed tables use zlib 1.2.7
201031 10:24:06 InnoDB: Using Linux native AIO
201031 10:24:06 InnoDB: Initializing buffer pool, size
=
8.0G supported file format is Barracuda.
201031 10:24:07 InnoDB: Waiting for the background threads to start
201031 10:24:08 Percona XtraDB ( www.percona.com ) 5.5.61-MariaDB-38.13 started; log sequence number 1966363829
201031 10:24:08 [Note] Plugin 'FEEDBACK' is disabled.
201031 10:24:08 [Note] Server socket created on IP: '127.0.0.1'.
201031 10:24:08 [Note] Event Scheduler: Loaded 0 events
201031 10:24:08 [Note] /usr/libexec/mysqld: ready for connections.

Answer 1

Someone restarts the server, look at the cron, maybe some unfamiliar programs are running, maybe something has been added to the systemd database, there is no fall in the logs.
It's very interesting where so many root sessions come from