[SQL Server] Database Mirroring login attempt by user ‘DOMAIN\SERVER$.’ failed with error. ‘Connection handshake failed. The login ‘DOMAIN\SERVER$’ does not have CONNECT permission on the endpoint. State 84.’.

When setting up mirroring (and witnessing) among SQL Server instances when the servers are configured using Active Directory it will automatically try to auth using the ID of the server. You need to grant CONNECT permissions on the endpoint on each server for the other server that will be connecting to it (or two if […]