Strange IE and FF issue

I have one PC that can connect to https://domain.com:81 with no problems and another that won't. I have had this problem since day one of the install. Both PC's are running the same versions of IE and FF. I have the firewall on both PC's off so no ports are being blocked from that. Any other ideas where to check?

 

Are both PCs in the same local network?

 

After a reboot if works now. Very strange but I won't complain.