We've been notified of a problem with Mumble client packages 1.2.2 and 1.2.3 released before January 15, 2012. If you try to connect to a server with such a client, you'll receive a message stating "A referral was returned from the server". If you receive this error, download an updated 1.2.3a package. Be sure to uninstall your previous 1.2.3 client before installing the new 1.2.3a client.
You can download the new installer from here: http://sourceforge.net/projects/mumble/files/Mumble/1.2.3a/mumble-1.2.3a.msi/download
For more information, see this post on the official Mumble blog.
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