This error occurred in my environment when I tried to create a database dump. This happened after I updated the MariaDB from version 10.2.x to 10.3.x. After upgrading your MySQL you have to run mysql_upgrade to fix any incompatibilities in System tables.
Just run the below command at your Plesk command line after upgrading your MySQL, restart MySQL and it will fix the issue:
Upgrade MySQL databases:
MYSQL_PWD=`cat /etc/psa/.psa.shadow` mysql_upgrade -uadmin
Restart the MySQP service
systemctl restart mysql