I might have found a solution but am not sure whether that makes sense. I have the products_Results.php and product_Details.php pages in a sub-folder and did also copy all checkout pages into this sub-folder (since I received an error in this regards before). However, I now did also copy all checkout and cart pages to the main root which seems to take care of the error.
So I now see the order confirmation page and have a few other problems such as shipping not being displayed and an internal server error message after a while of loading. However, the order seem to have gone through since I receive an authorizenet confirmation (Test Mode - not sure why this is since I did not choose test mode). Anyway, I guess I will test this a little further and search the forum to better understand specifically what the problem is before I bother you guys again.
Unless you would have any ideas :-)
Thanks a lot.