When trasnferring (migrating) subscriptions with mail accounts following errors occur in Parallels Plesk Panel > Tools & Settings > Backup Manager :
Error: domain "domain.tld"
Could not find password for account id '1831'. Return empty password
Info: mailname "firstname.lastname@example.org"
The password was generated for user 'email@example.com'. New password is 'Iji2OUq'
An empty password for the mailbox
firstname.lastname@example.org on the source server. This is considered a software issue. The internal request ID is PPPM-1617 .
Update the password for
email@example.com on the source server in Parallels Plesk Panel > Domains > domain.tld > Mail > firstname.lastname@example.org .