Trying to change the FTP username for any domain fails with the error:
Error: ProtDirUser_IIS::update() failed: Update Protected Directory #308 user failed: Url 'plesk-stat' is not protected. (Error code 1)
Virtual hosts structure has not been properly updated during an upgrade.
Delete all the password protected directories
Open "Command Prompt" (Start > Run > cmd.exe) and execute the following commands:
cd %plesk_cli% repair.exe --update-vhosts-structure repair.exe --update-all-domains-web-stat repair.exe --synchronize-impersonation-storage