ssh -X: can't open display

[tospo]

Hi,

I know variations of this question are asked all the time but somehow I couldn't find a solution for my problem anywhere so I'll just give it a go here:

I am simply trying to get x-forwarding in ssh to work on my newly installed Kubuntu box and I get the infamous "can't open display" error.

I know that the remote machine (Centos server) is correctly configured for x forwarding because I access it all the time from yet another machine (Debian) and it works.

When I use this command on the Kubuntu machine

ssh -X USER@REMOTEHOST

I can log on to the Centos machine but when I launch an application like xterm on the remote machine I get:

xterm Xt error: Can't open display: client-xx-xx-xx-xx.cmbg.adsl.virgin.net:0.0

Virgin.net is my ISP and I am using a Netgear DG834 router. I thought that the router is blocking the port but even when I allow all inbound and outbound traffic in the router's firewall settings I get the same problem. I also tried

xhost +

on the local machine and that too didn't have an effect.

Could it be that my ISP is blocking this or is there anything else (probably blatantly obvious to anybody but me ) that I have overlooked?

I should probably mention that the Debian box from which the x-forwarding does work is not on my home LAN and also not on the same network as the Centos machine to which I need access. I hope this makes sense....

Thanks for your help!!