Internal Webserver Login Failure

We have recently switched out our existing router for a Routerboard which we use quite a bit at other locations. At this site we have an internal webserver which requires a login (username/password). The site can be accessed internally without problem, and externally without problem using the old router. I have forwarded incoming traffic on port 80 to the internal IP address of the server. When I attempt to access the site externally, I get the main page which asks for the username and password however after I enter the credentials and try to proceed, I receive an error login failed.

I suspect that’s because i’m coming back on a different port other than 80, but i’m not sure. From the connections window, I can see my connection open and close immediatley when hitting the main page and no other connection created when the login is attempted. In the firewall I have rules to permit related and established connections. Any help is greatley appreciated.

Brandon