Thanks for your answers!
There are a couple things to check first is the firewall
This is inside the same LAN, should've said that.
I'm using firestarter in the remote box, but open for the client's IP in that port.
No logged blocked events light up in Firestarter.
then the protocol there is ssh1 and ssh2
Only protocol 2 in the server (in /etc/ssh/sshd_config). In the client, I choose ssh2 in gftp (which I assume is Protocol 2).
if ICMP is denied in the firewall ping will fail.
It's not denied in Firestarter. Ping is denied in the router, but it seems to make no difference for access from within the LAN.
Next try pinging the host from the client to test the connection
Pings just fine.
Even an SSH connection from the terminal starts completely fine. The problem seems to be restricted to sftp (within gftp, with the option "-s sftp").