Internal server web access not working

I have OS X Server 10.2.3 and a newly installed firewall. Prior to the firewall I had a Draytek router that worked great but I needed additional protection and now I have a Watchguard SOHO6 Firewall with routing capabilities. With the Draytek router I could access my Web Server internally, as well as over the internet. Now after the installation of the firewall I myself can not access my web server. It is only accessible over the internet externally. To make this even more complicated, I can not access it at all from my internal LAN over the internet. When I remove the Watchguard firewall and replace it with my older Draytek router, it all works again. Watchguard does not know why and I am convinced it had something to do with the server setup. I have heard cases like this before and know there is a solution. I need some help with this and afp548 is definitely the place to ask. So come on and let the tips and hints fly.