Symptoms

• Cannot export MS SQL dump in Plesk (Subscriptions > example.com > Databases > Export dump):
  

  Unable to export a dumps: Unable to backup database 
  Unable to open network connection: The user name or password is incorrect (Error code 21)

• Changing password for the database user does not take any effect.

Cause

The password specified for the MS SQL user is invalid.

Resolution

Update user credentials for MS SQL server in Plesk:

1. Go to Tools & Settings > Database Servers.
   
2. Click on the host name of needed MS SQL server:
   
3. Go to Settings and specify a new password under Authentication section.