I worked around the problem like this:
- I open the dialog window so the editor gets initialized
- I then start a recursive loop with window.setTimeout (200 milliseconds per cycle) that checks if the method editor.setHTML is defined (typeof(editor.setHTML) == "function")
- If it is defined (which will always be the case for IE) the editors' value is preset
- If it is not defined it just runs the cycle again
It works for any browser now, but it is far from a good solution...