Mumble uses certificates to match users to their registered names. This ensures only the proper user can login with that name. Because of this security feature, if you lose or change your certificate, you will no longer be able to login as your username and instead get the message, "Wrong Password for Registered uesrs, Please try again".
To resolve this, the user would need to ensure their certificate the Mumble client uses matches the one on the Mumble Server; if they have no backup of their certificate to revert to, you can resolve this by logging in to Mumble as the SuperUser and going to Server -> Registered Users and then un-registering the user who lost their certificate. Next time this user logs in to Mumble, they can re-register the name and everything will work.
To prevent this from occurring, users should make backups of their certificates. This can be done from within the Mumble client by going to Configure -> Certificate Wizard -> Export Current Certificate.