This is truly a decent and instructive, containing all data furthermore greatly affects the new innovation.

The behavior of shutting off and restarting when the switch is activated is usually a wiring problem - most often an incorrect neutral. The switch is losing power - which of course does not affect manual switches. When you checked the wiring did you confirm that each wire is what you expect by measuring or only by verifying the color? You may want to double check, as depending on how the fixture is wired the colors can be misleading.

I'm having the same symptoms with an HS210 installation that I did earlier today with just one of the switches (the second switch doesn't have a neutral available, so I left it untouched with the original hardware).

The app never finished because of the missing second switch. HOWEVER, the first switch seems to have been administered correctly, it connects to the wifi, and I can see it in the app. It even pulled in a firmware update.

Can I confirm that the flashing/resetting behaviour is not related to the app issue with the missing second switch?

My gut tells me that the HS210 is too small for the box that I installed it in, and that it may be grounding out to the box. Would I expect the flashing/resetting behaviour if this were the case?

I'm also fairly new to electrical work, so one of the sanity check questions I have concerns the metallic contact/"washer" that sits between the screw and the screw hole. I wired the hot and two travellers between the screw top and the washer. Is this the expected configuration? I'm expecting 'yes'... that it's the screw itself that's conducting. Just trying to eliminate potential issues.

Thanks for your time!

Just a quick note about how my situation turned out.

It looks like I'd wired the two switches properly, and that the circuit matched the simple 3 way circuit, minus the neutral on the second switch.

The problem was with the light itself. The wiring is/was a complicated mess that I still haven't figured out. But it's definitely not a simple 3 way circuit. I'm not really chasing this one down, but just sharing details for the sake of trivia: the light has three bundles of three wire (hot, neutral, ground). But I don't see the red traveller wires from the two switches anywhere... I can understand that they *could* just run directly between the two switches, but then that means the three bundles going into the light are not directly linked to the switches (so where do those three bundles come from, then?)... and I'm not sure what would be accomplished by running the bundles with the travellers directly/uninterrupted between the two switches.