I bought a HS210.
The configuration is Line -> Regular 3-way (Position A) -> HS210 (Position B) -> Load
If I connect the HS210 in a pair with a regular 3-way switch, the configuration works with no problems.
If I connect a regular 3-way switch in Position A and a Lutron Dimmer Switch DVCL-153P in Position B, the configuration works with no problems.
If I connect the HS210 in Position A and the Dimmer in Position B (with light at 100% level, i.e. not dimmed at all), the configuration doesn't work and the HS210 starts clicking turning lights on and off, etc. Things get worse if I start dimming. Oddly, if I connect the neutral directly to the ground wire, everything works (I haven't tried dimming in this configuration today, but I think it worked when I tried yesterday). I am fairly sure that the wiring is connected correctly (however, the load neutral, line neutral, and HS210 neutrals are wired together with a single wire nut - I have no choice).
I also tried the configuration: Line -> HS210 -> Load to use the HS210 as a regular switch. I kept the same neutral and ground, connected the power to one of the travelers and the load to the load/line screw. That worked as expected.
If I put the HS210 in Position A, everytihng I do works as expected.
So, the question is simple: am I doing something wrong or HS210s don't work with a regular 3-way dimmer when it's in Position B? I understand that, in this case, the HS210 is powered by the travelers and, hence, its voltage supply isn't the cleanest, especially when you start dimming).