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