I have a fresh installation of the latest version, I don't have any firewalls, and I was able to connect yesterday. My brothers can both connect (we're on a network) but I can't. I tried hosting my own server which I was able to connect to, but they wern't able to connect to me. I'm not sure if this means it's a problem with my computer, or if something was just wrong with the way they were trying to connect to me.
Anyways, I was able to connect to the lobby yesterday, but I can't connect today. I haven't made any changes to my computer inbetween the time I connected yesterday and today.
