We have planned to remove the support for the relay feature: in terms of hardware the new OS units do not have the built-in relay any more; and in terms of software, the relay support will also be removed at some point (although the current firmware still supports it).
Since you can hear the relay click, that means it’s activating correctly. Also you said manually shorting the two wires triggers the garage door action, so those wires are also fine. This leaves only two possibilities: soldering joints, and the relay itself. It’s possible that the relay is defective, so while the coil can activate, the contact points no longer connect reliably. You can use a multimeter to do continuity checking of the two relay pins (that connect to your garage door wires). When the relay is activated, the two pins should be shorted, simulating shorting the two wires.
Do you know how long the relay clicks each time you activate S8? It’s supposed to be a pulse (i.e. the relay should turn on for a short amount of time and turns off). The timing can be controlled by the ‘Relay Pulse’ option (under Advanced Tab in Edit Stations page). If it clicks too briefly, it might not be sufficient to trigger the garage door. If it just stays on, that might also cause issues. I think 2000ms is a good number to try.