MODx "Invalid XML response from connector" error

This is an annoying error that you can get in MODx when uploading new files via the FCK Image Browser, and I've only ever seen it in the Internet Explorer browser. The fix for it is quite simple though...

1. Open manager/media/browser/mcpuk/connectors/php/Commands/GetFileUploadProgress.php
2. After //Progresshandler not specified, return generic response (mine is line 46) insert:

   header ("content-type: text/xml");
   echo "< ?xml version=\"1.0\" encoding=\"utf-8\" ?>\n";

3. See if that works

If that doesn't work then here is the MODx forum post that goes into more detail. A variety of problems can cause this issue (like the file being larger than the allowed max size).