Note that ThinLinc connections are a two-step process. First the client establishes a connection to the ssh address you state in the connection form. But once connected, the ThinLinc server replies with the address where the client should actually connect to. In a simple setup this is the address the ThinLinc server itself thinks it has. So it is important to make them match up.
You can set the ‘address’ using the follow command
tlcfg set-hostname external-name-of-tl-server