security error 1000 - editing area - event handling

Dear fckeditor 2.6.x users,

I want to change the text in the editing area every time a users keys in, say 'A'.
I adopted sample08.html from the '_samples' directory.

The essential part of my code looks like this:

...
   function FCKeditor_OnComplete( editorInstance )
   {
           if (document.all) {
                // IE
             editorInstance.EditorDocument.attachEvent("onkeyup", keyboardHandling) ;
           } else {
              editorInstance.EditorDocument.addEventListener( 'keyup', keyboardHandling, true ) ;
           }   
   }



   function keyboardHandling( ed ) {
   ...
      try {
         var old = ed.GetXHTML( true );
                         old = "*" + old; //just to see that something has changed here
         ed.SetData(old); // SECURITY EXCEPTION
                        alert('ok - passed'); // this line is never executed!
...
      } catch(err) {
        alert("error: " + err.message);
      }
      
      ...
   }

So my question is, where does the security error 1000 come from and how can I get a workaround for this problem? The editing area is no more responding after a keystroke, so the source code is useless.

Any ideas? Pleas help.

Best regards
toller43