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 If such an outdated Mumble is launched after that date the error message “A referral was returned by the server” will be displayed by Windows and Mumble will not start.
You can get the fixed Mumble 1.2.3a stable installer from: http://sourceforge.net/projects/mumble/files/Mumble/1.2.3a/mumble-1.2.3a.msi/download (uninstall 1.2.3 stable first). An updated installer for the snapshot version is available from: http://mumble.info/snapshot/mumble-1.2.3-271-gca8b25c.msi You can also download a fixed Mumble.exe for the 1.2.3 stable from: http://sourceforge.net/projects/mumble/files/Mumble/1.2.3a/mumble-1.2.3-resigned-binary.zip/download
We are sorry for the inconvenience this may cause. We will follow up with a more detailed explanation later.
Regards, The Mumble team