SymptomsMigration has been completed with the errors:
An error occurred during mailname update: Unable to update the mail name
properties: The password is incorrect, use at least 5 characters."
ResolutionMostly the problem is not in the password, but in "Use of short (webmaster) and full (email@example.com) POP3/IMAP mail account names is allowed" option on 'Server' -> 'Mail' screen in Plesk CP.
While this setting is enabled, it is not possible to create two mail accounts with the same local part (like 'sales' in 'firstname.lastname@example.org') and two identical password OR if one or the both passwords are encrypted because encrypted passwords cannot be compared.
As solution you can set "Only use of full POP3/IMAP mail accounts names is allowed" and try to create mail account or perform a migration again.
After that you can find all duplicated mailnames/passwords with the script that is attached, resolve all conflicts and change "Only use of full POP3/IMAP mail accounts names is allowed" setting back if you need it.