Aside from the usual incorrect server address/port problems, there are two common problems people run into. Also, almost all mumble servers are self signed. So if you receive the "This certificate is not trusted" message, it's usually okay to accept it anyways. On that topic, for murmur administrators, Mumble wrote up how to get a free signed cert [here](https://wiki.mumble.info/wiki/Mumble_Certificates)
Problem: This user is already registered This happens when connecting to a server with your username, but from a new or reformatted computer.
Solution : Use a different username or ask the mumble admin to delete your user, so you can re-create it.
What Happened: When you register your username, it saves the credentials on your computer. This is all done in the background. When logging in from another computer, the credentials are not found. If you want to sign in from multiple computers you can go to Configuration -> Certification Wizard and export your current cert.
Problem: Continuously pops up the same window then fails Sample error message "Sever presented a certificate which failed verification.."
Solution: For Windows 7 go to %AppData%\Mumble_ For Windows XP go to %localappdata%\Mumble Warning_: This next step will delete all your saved connections and certificates._ Backup and Delete the .sqlite file, a new one will be generated and it should work.
What Happened: It looks like the mumble sqlite database got corrupted