Mumble worked fine until I tried to log in today. I can connect to all public servers and one of my three favorites, but not the other two. When I try to log into them directly through the Windows 7 start menu, I just keep getting "server connection failed: host not found". If I open Mumble and press server>connect, the list of my three favorites is there, but the "Connect" button is grayed out for both the servers I can't connect to. The other still works fine, and like I said, otherwise everything is fine. I know the servers are working because people are telling me they're in them right now.
I've tried rebooting all hardware, reinstalling Mumble, etc. I can't connect to either of the two, and I need to use them soon. The administrators of the servers have no advice left to offer.
Had the same issue, I was able to connect after using the IP address instead of the server name under the 'Address' field. Try that and see if it resolves it. Good luck!
The exact same problem happened to me today, did you ever find a fix?