Such problem happens when the password of one of two/several identical mail accounts is stored in Plesk database in encrypted view. As it is encrypted, Plesk cannot compare passwords and make sure that passwords are NOT identical. That is why Plesk does not allow to switch back from 'full' to 'short' mode and does not accept any password if you have 'short' mode set.
The solutions are:
- either continue 'full' mode usage
- or find encrypted passwords and change them through Plesk GUI (they will be saved in plain-text view in Plesk base). For example, to get the list of mail accounts and their passwords, you can use the following command:
# mysql -uadmin -p`cat /etc/psa/.psa.shadow` -Dpsa -e"select m.mail_name, d.name, a.type, a.password from domains d, mail m, accounts a where m.dom_id = d.id and m.account_id = a.id"