When I joined a Google Hangouts video conference with Google Chrome, from time to time, I get the error message
Couldn't start the video call because of an error"
Please check your firewall settings and try again.
I use the same computer (OS: Windows 7 SP1 x64 Ultimate) and the same network (Ethernet, on a University campus: 1 Gbps symmetric, ~3ms ping): the error sometimes appears, sometimes it works fine.
I don't have any firewall.
What could the issue be?