Hi franen,
You could try using the latest cifs version and working backwards until you find one which the NAS supports. The later versions are also more reliable and faster.
The cifs versions which could work are (oldest to newest):
vers=1.0, vers=2.0, vers=2.1, vers=3.0, vers=3.02, vers=3.1.1
So the mount options that you could first try
cache=none,noserverino,ro,vers=3.1.1
then work backwards through the list until
cache=none,noserverino,ro,vers=1.0
This has solved problems for a couple of other users (not using your NAS) and it is now impelemted as standard in Rune 0.5b.
janui