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

The certificate of mumble 1.2.2 has been expired. As a result, you must update your mumble to 1.2.3 a in order to continue using mumble on our server.

If you have troubles accessing our mumble server. Please do the following:

1. Uninstall mumble
2. Install mumble with the new mumble client available on: http://blog.mumble.info/important-update-to-mumble-1-2-3a/
3. Try to rejoin

Our server IP is hsp.hk:64738