Here is a solution:The server didn't send back a proper XML response.
1) Open [ROOT]/modules/FCKeditorX/FCKeditor/editor/filemanager/browser/default/connectors/php/connector.php ([ROOT] is your base CMS Made Simple directory -> normally it's called cmsmadesimple)
2) Find check_login(); (Line 26) and replace it with #check_login(); (You notice the new # before the function which will command out that line)
3) Save and upload the file.
Now everything should work!
Attention! This 'fix' removes the authentication for the file upload. It may be possible for other persons to upload files.