Can't log in to MailEnable webmail on Plesk for Windows: Unknown or Disabled User!

Symptoms

Trying to log in to a Plesk email account with MailEnable webmail fails:

Unknown or Disabled User!

Cause

The mailbox password was changed outside Plesk (i.e. on MailEnable admin console or webmail interface), and Plesk was updated or repaired after.
Passwords are replaced with the ones from the Plesk database during upgrades and mail repair; this overwrites changes made outside Plesk, leaving a mismatch in credentials.

Resolution

Reset the password using the Plesk interface.

1. Log into Plesk
2. Go to the affected mailbox in Domains > example.com > Mail tab > Mail Accounts > jdoe@example.com
3. Enter the password in the Password and Confirm password fields
4. Press OK to save the changes