I have seen this pop-up on a couple different forums and even though it is not IIS specific, it always seems that IIS catches the blame for it. The issue is that external clients have no problem connecting to a website that is hosted on an internal server, but clients on the internal LAN cannot connect. After troubleshooting through this on the Technet forums with a poster it comes to find out that the issue is the internal LAN clients are resolving the website name to the external IP. For some reason the router is not allowing these internal clients to connect back internally correctly.
We came to that conclusion after I had him add the domain and internal IP address to his lmhost file on the client PC. When that was in place the website would correctly resolve for the LAN client that had the change. To clear this up he setup a DNS record on his internal DNS servers for the website using its internal IP address so the LAN clients would not have to go through the router to resolve.
I didnt really think much of it when I first worked with the poster, I was just happy to help, but since then I have seen it come up on at least 3 different forums and 1 twitter, hopefully this will come up with a relevant search and maybe help resolve the issue quicker for someone looking for an answer.