Problem/Error
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Plesk Update Fails- MYSQL Error

When Plesk update itself, it connects to local MYSQL server. If it can not connect, then update fails. Most of the time it happened because of wrong credentials.
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Solution

1. Check if you can manually connect to local MYSQL server.
"%plesk_dir%Databases\MySQL\bin\mysql.exe" -uadmin -p<type password here> -hlocalhost

2. If you can not connect then you can reset password using following command

"%plesk_bin%\dbclient.exe" --direct-sql --sql="Update databaseservers set admin_login='<MySQL user name>', admin_password='<type password here>' where host='localhost' and type='mysql'"

After you you reset password, again go to step `1 to check whether you can connect to MYSQL server, if you can then next time error won’t appear when you update.

Done!