Cannot manage users/mail accounts through web interface.
Error in Panel looks like:
Unable to find the user account. It was probably removed by someone else while you were changing its settings.
The e-mail address was not found. This can happen if e-mail address was removed by someone else while you tried to access it.
This issue is caused by incorrect owner of reseller's user 'Reseller1' (from smb_users) which belongs to another client, instead of reseller.
Backup Plesk database:
"%plesk_dir%"\MySQL\bin\mysqldump -uadmin -p"*****" -P8306 psa > psa.dump
Update database with next MySQL query:
UPDATE smb_users AS smb INNER JOIN clients AS cln ON smb.login = cln.login SET smb.ownerId = cln.id ;