[CS3] [JS] User canceled

I am doing my first steps trying to make the InDesignServer to create some pages. For some reason I sometimes get an error message: "User canceled this action."
Obviously there is no user involved. And nothing is canceled. I cannot figure out what causes this problem.
I trigger the server JS script from applescript passing some arguments. But I also had this error running the script directly from ExtendScript Toolkit 2.
Anyone experienced something like this?

Thank you,
Ralf