Database backup in Plesk fails because of missing MySQL/MariaDB user: The user specified as a definer does not exist

Symptoms

• A backup task finishes with the following warning message in Plesk:

  Warning: mysql "dbname"
  Not all the data was backed up into /var/lib/psa/dumps/domains/example.com/databases/dbname successfully. Total space: 18.00 GB; Available space: 11.00 GB; Mounted on: /. mysqldump: Got error: 1449: "The user specified as a definer ('old-dbuser'@'%') does not exist" when using LOCK TABLES

• When exporting a MySQL/MariaDB database at Domains > Databases > dbname > Export Dump, the operation fails with:

  Unable to export a dump of dbname:
  mysqldump: Got error: 1045: "Access denied for user 'dbuser'@'%' (using password: YES)" when using LOCK TABLES

• When opening views in PHPMyAdmin, the page fails to load with:

  #1449 - The user specified as a definer ('old-dbuser'@'localhost') does not exist

  ---

  #1045 - Access denied for user 'dbuser'@'%' (using password: YES)

• Migration of a subscription via Plesk Migrator fails with the following error message:

  Failed to copy content of database 'dbname'
  Migration tools tried to perform operation in 3 attempts: Command execution failed on the source server 'source' (203.0.113.2) with non-zero exit code.
  command: MYSQL_PWD="$(cat)" mysqldump --no-defaults -h localhost -P 3306 -uadmin --quick --quote-names --add-drop-table --default-character-set=utf8 --set-charset --routines --events database_name > /root/plesk_migrator/plesk_migrator-b1i6noaid74taessfxtzkzxsliabvk0z/db-dumps/dbname.sql
  exit code: 2
  stdout:
  stderr: mysqldump: Got error: 1449: The user specified as a definer ('old-dbuser'@'%') does not exist when using LOCK TABLES

Cause

The MySQL/MariaDB user set as a definer in the MySQL/MariaDB view is missing on the system or has different  "Access control" setting.

Resolution