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: