I had this error too.
After debugging a litle the java script in browser.html, I found out the editor is calling this URL:
http://localhost:1923/controls/fckeditor/editor/filemanager/connectors/aspx/connector.aspx?Command=GetFoldersAndFiles&Type=Image&CurrentFolder=%2F&uuid=1252103392107
Once I requested this URL directly I got this error:
Using themed css files requires a header control on the page. (e.g. <head runat="server" />).
So in my case, the error was caused by my application using themes.
What I did to solve this was adding this line:
<head id="Head1" runat="server" />
At the begining of
fckeditor\editor\filemanager\connectors\aspx\connector.aspx
That fixed it and now it's working just fine. Hope it helps