I had the same problem with WHS1 and the problem is that your router does not support loopback. All you have to do is add a line in your HOSTS file in your PC (not server) with the server's IP address and server domain name, such as this.
#Start of Homeserver#
192.168.0.2 xxxx.homeserver.com
#End of Homeserver#
The HOSTS file resides in Windows/system32/drivers/etc. I always copy it to the desktop edit it with notepad and copy it back overwriting the original.
Colin