As TheFu suggests, it is probably a DNS issue. Here's one way to test. One one of the local machines add an entry to its hosts file that links the server's local address to your domain name. If the server has address 192.168.100.50, you'd add an entry to the hosts file like this:
Originally Posted by rebeltaz
If that resolves the problem, you'll either need to add an equivalent entry in every machine's hosts file, or set up an internal DNS server.
Oftentimes routers, especially ones designed for residential use, have a problem with traffic from internal hosts that is intended for another internal host but addressed with the router's external address. For example, suppose the router's public IP address was 10.10.10.10, and its port 80 was forwarded back to 192.168.100.50:80. From the internal network, you should be able to connect to the latter address, but connections to 10.10.10.10 may fail.
(I just worked with a client who was switching to a new ISP, and that ISP provided a router that had this exact problem. After we tore our hair out for a while, we changed the target addresses in the organization's local DNS server to point to the internal addresses, and it all worked as expected.)