I have the exact same issue as well!
It only seems to affects Android phones using IMAP, it tries,(and fails), using DIGEST-MD5, then works on the second try, but after a short amount of time, the fail-to-ban has been triggered.
Still no solution?
I have the exact same issues and error messages since May 5'20.
My server is:
Ubuntu 16.04.6 LTS
Plesk Obsidian
Version 18.0.26
On May 4'20 my Plesk automatically updated from 18.0.24 to 18.0.26 and now "EVERY" attempt to do an FTP backup,(scheduled and manual), fails!!
The backup...