Problem with app.openDoc in 8.1

In Acrobat 8.1 WIN XP the following command does not work (it worked in 7):

var otherDoc = app.openDoc("/c/temp/myDoc.pdf");

Error Message: NotAllowedError

I use this command directly when opening a PDF-document. I also tried a "trusted function" with the same result: it works in 7, not in 8.1.

The command works when I put it in a button.

What do I want to do? When opening a PDF i want to open a second PDF.

Question: How do I use app.openDoc when I want (during opening of a PDF) to open another PDF in 8.1?

I also tried different (safe) paths with the same result

Martin