Can't connect to a specific friend's server and they can not connect to mine

[DogofGrog]

Been trying to play netplay with 4 people. When I host, my friend presses "Connect" on Dolphin and it does nothing. The same happens to me when he hosts. When friend 3 hosts, however, we both can connect. The problem whether its a transversal server or direct connect server.

• All our setttings are correct
  
• We've port forwarded
  
• We've uninstalled/reinstalled Dolphin and ROMs
  
• Its not a Windows Firewall issue
  

Any way we can fix it?