Thanks Frank 
Well, I can only assume it’s defective as I changed nothing other than the relay itself and then all worked as it should 
I have the same wiring. I am stuck with the line (in) at the bottom of the stairs with switch 1, the load (out) at the top of the stairs with switch 2 and the light in the ceiling at the top of the stairs with a 14-3 (b/w/r/g) cable between 1 and 2. I have no way of installing another conductor to get the line (in) to the top of the stairs. Would it work to convert the two 3-way switches into single-pole, “steal” the traveler (red) wire and make it hot to the top of the stairs, tie the red to Sonoff L in, and to the switch 2 line and connect load to Sonoff S2, use black wire at switch 1 as load to top of stairs into Sonoff S1, and then the black to the load (out) into Sonoff L out? Or does that create an overload problem with potential fire hazard?
Follow-up on my earlier question, in case it helps someone else with the same issue.
Setup: two 3-way (staircase) rocker switches controlling one light, in separate junction boxes about 7 meters apart. ZBMINIL2 installed in the box next to the bulb.
What DIDN’T work:
Wiring each switch as a simple 2-terminal switch (common + one traveler-turned-signal-wire to S1 or S2 respectively), leaving the third terminal on each switch unconnected. Result: each switch toggled the light perfectly on its own, but combining them failed — e.g. turn the light ON with switch 1, and switch 2 couldn’t turn it OFF. Wiring was triple-checked with a multimeter and was correct; the issue was that with one terminal left floating, each switch only produced a defined electrical signal in ONE of its two rocker positions.
What DID work:
Keep the ORIGINAL 3-way wiring between the two switches completely intact — both switches keep all 3 terminals in use (common + the two traveler wires between them), exactly as in a traditional staircase circuit. Only the COMMON of switch 2 connects to S2 on the module. S1 is left unused entirely. The module’s L in gets a permanent live feed (in my case from a nearby socket circuit, clearly labeled since it’s a different breaker than the lighting circuit — worth avoiding if you can feed it from the lighting circuit instead).
This matches the wiring diagram shared earlier in this thread (switch loop → S2 only, L in fed separately).
Official confirmation:
I contacted SONOFF support directly and they confirmed: S1 is not used in 2-way (staircase) configurations. Only S2 is used, fed by the intact traveler loop between the two switches.
Hope this saves someone else the troubleshooting time!