Hi,
Thank you @Andreas Grassl for your input.
The client I was visiting before just was lucky to notice the infection
in time, so the NAS wasn't encrypted, only his desktop pc.
The files on dropbox he should be able to recover using the versioning
feature of dropbox.
So, sorry, I can not provide any more useful hints.
Update on my situation: Digging deeper in the details of the
TeslaDecoder provided at bleepingcomputer.com and factoring the Public
Key with the help of the Forum users gave me the private Key necessary
to decrypt the files, at least the most important ones needed for the
moment.
Good luck,
Cheers,
ando