SymptomsIn Plesk Panel 10.x, you must define the label for messages that are defined as spam. Empty values of the input field (" ") are considered as invalid.
ResolutionReplace AntispamTab.php with the attached file. The new php file will remove the restriction of sending empty values for the label of a spam message.
This problem has been fixed in Parallels Panel 10.4. Consider upgrade Parallels Panel on your server.