Hello Chris,
I may have not explained it well on the Live Chat, please let me explain.
When the issue was reported on the Chat/board, one of the members from technical department got involved as neither Jack or I was available.
Now Jack and I have made a few manual changes (in order to overcome few Plesk limitations) on your
VPS Hosting that are not in the PSA database. The person who got involved had no knowledge about the changes that were made and the last thing that he did, was to run a utility that is provided by Plesk to set a domain on the default settings, which removed all custom settings including the 404 Error Page setting from IIS Web Server.
I am not trying to defend any one but just to say that whatever measures that one took was in direction to help you & get your issue sorted.