Not sure if this will make a difference for you (it's been a fix for Vista users with ZA who have had probs) but you can try if you like...uninstall both LW & ZA. Reinstall LW first, then reinstall ZA.

You could also try port forwarding for your modem (it has an internal firewall). The instructions are here.
Port Forwarding for the Siemens 4100
Don't forget the step of setting up a static IP address first. LW's listening port, port in the 'manual port forward' box & port forwarded within the router should all be the same number.

You could also try the version of Java that I linked to in my other post (1.5.0_11).