"Due to an expiring certificate no Mumble 1.2.2 or newer released before today (15.01.2012) will work on the Windows Platform after 15 January 2012 11:36:47 GMT. If such an outdated Mumble is launched after that date the error message “A referral was returned by the server” will be displayed by Windows and Mumble will not start."

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http://blog.mumble.info/important-update-to-mumble-1-2-3a/