Problems adding bookContents to a book

Hallo,

i have a simple script adding an empty book to app and then trying to add some bookContents to this book.

Whenever i try bookContents.add() i get an error reading "Referenzfehler: Vorgang vom Benutzer abgebrochen." - "Reference Error - Process aborted by user".

What's wrong with this script? Are there any user operations possible in Indesign Server? Please help!

Code:
bookFile = "C:/book.indb";
var myBook = app.books.add(File(bookFile));
var myBookContents = myBook.bookContents;
file = "C:/test.indd";
myBookContents.add(File(file));

Thanks in advance
Thilo Machill