Good idea!
Telnetting from the .11 address works perfectly fine (when using HTTP/1.0 ... HTTP/1.1 results in a bad request).
The port 81 thing is due to the NAS' admin interface which has a second webserver listening on port 80 (a behaviour that cannot be changed as far as I can tell).
I had also thought of a port issue earlier, but changing the Arduino code to fetch the admin page on port 80 also doesn't lead to a positiv result. Is there any other piece of code that I could try to check if the Arduino and the NAS are capable of talking to each other? The NAS not only provides samba shares but also a MySQL database as well as a Twonky UPnP streaming server.