Starting from today most of you, well actually probably everyone, will have a problem when trying to use Mumble. It will not start and output an error message.
This means that you must update Mumble to 1.2.3a. The error is due to a expiring certificate. For more information just click on the following link.
http://blog.mumble.info/important-update-to-mumble-1-2-3a/