Yesterday after rebooting once (when I wrote into the forum), I was still 2 more hours on the PC with the NAS on, also copying stuff to it, and the connetcion stayed.

I noticed since some months that my modem has some problems holding the connection, I'm just not sure if it's the modem itself, or just my ISP. I have another new modem which I got with my DSL package, but changing to it would mean to change all my IP addresses, as my Fritzbox uses 192.168.178.... and the other one uses 192.168.1.... I have like 8 different devices with IPs connected, software linked to it, port forwarding and file sharing, I don't want to fiddle around and change everything manually now.