After 1 week encountering the “Unable To Join Game” error on battle.net, finally I have found the solution to fix this.
First, run “Resource Monitor” system application of Windows, and go to “Listening Ports” section, click to sort by “Port” tab and then you can see which application is running on port 6112 - this is the reason why battle.net can’t use this port for joining/creating game on battle.net.
So the solution is killing and then quarantining the app which is running on port 6112. In my case, that app is “ABService.exe” - an auto schedule service from AOMEI backup app.
Also, you should go to game option - network, check to “Prefer 6112 port” and NAT/Port forwarding 6112 both TCP/UDP on the modem/router.
Hope this helps. Good bye.