When I hook up one HS210 and leave the standard 3-way in place, when I try to turn on the lights with the switch, my lights flash and the HS210 makes a clicking noise. This repeats until I either disconnect the HS210 or turn off the circuit breaker for this circuit. I have had this problem before in this house. The problem seems to be that the two white wires tied together in the box were not actually Neutral wires. I had connected the Neutral from the HS210 to these two non-neutral white wires. As soon as I removed the Neutral wire coming from the HS210 the lights flashing and the clicking noise from the switch stops. Of course, at that point I was unable to control the lights .

I then did a test and plugged in an extension cord into an outlet and inserted the Neutral wire into the Neutral socket (wide slot) of the extension cord and the switches work as expected - no more flashing lights, no more clicking, and, I am able to control the lights from either of the switches.

Apparently, the electrician who wired my home did not run Neutral wires to all of the switch boxes. I don't know if this was in accordance with Code or not. 

The long term fix, I guess, and, I am not sure it is code is to run a neutral wire to the switch box from an outlet box especially if they are on on different circuits, but it did fix the problem for me.