Hi Thorsten,
Could you please put here screenshot of your synology config & your nethserver cionfig as i have the same problem as you but didn’t manage to get it work.
When i clic on “Cheching” the parameters on Nethserver i have this in /var/log/messages 
Jan 26 17:04:52 neth cockpit-bridge: No entry for terminal type “unknown”;
Jan 26 17:04:52 neth cockpit-bridge: using dumb terminal settings.
Jan 26 17:04:52 neth cockpit-bridge: Permission denied, please try again.
On my Syno I activate SFTP, RSYNC and SSH all on port 22.
On my Nethserver i did :
Thanks all.
Happy New Year.
PS: [Closed] Do not put a “V” but a “v” on Synology volume1 name … 