Cannot install Plesk updates: ERROR while trying to disable old_passwords=1 in my.cnf

Symptoms

1. Plesk updates installation fails:

CONFIG_TEXT: ===> Preparing Plesk database upgrade (revertable stage).
ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2)
ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2)

ERROR while trying to disable old_passwords=1 in my.cnf

2. Errors after mysql service restart appear in /var/log/syslog:

CONFIG_TEXT: Feb 19 10:50:39 server kernel: [1068000.633468] audit: type=1400 audit(1519037439.725:172): apparmor="DENIED" operation="open" profile="/usr/sbin/mysqld" name="/proc/25965/status" pid=25965 comm="mysqld" requested_mask="r" denied_mask="r" fsuid=111 ouid=111
Feb 19 10:50:39 server kernel: [1068000.667549] audit: type=1400 audit(1519037439.761:173): apparmor="DENIED" operation="open" profile="/usr/sbin/mysqld" name="/sys/devices/system/node/" pid=25965 comm="mysqld" requested_mask="r" denied_mask="r" fsuid=111 ouid=0
Feb 19 10:50:39 server kernel: [1068000.704241] audit: type=1400 audit(1519037439.797:174): apparmor="DENIED" operation="open" profile="/usr/sbin/mysqld" name="/proc/25965/status" pid=25965 comm="mysqld" requested_mask="r" denied_mask="r" fsuid=111 ouid=111 

Cause

Apparmor blocks access to MySQL server files.

Resolution

1. Login to the server over SSH

2. Disable the Apparmor profile mentioned in the error message:

# sudo aa-disable /etc/apparmor.d/usr.sbin.mysqld

3. Restart Apparmor service:

# service apparmor restart