@ESokoloff : thanks for your detailed investigation.
I previously used
this mechanical timer, but I found that it was not cutting the power cleanly (once I was close to the bike at that moment, I heard the bike doing strange noises. I do not remember exactly, but it was probably the same buzzing noise as with the smart plug). And in last July,
the controller of my bike died after a charge stopped by this mechanical timer. I suspected the mechanical timer to be at the origin of this failure. That is why I decided to stop using it.
I do not understand why cutting the neutral instead of cutting the hot leg or cutting both would disturb the Zero. But it seems that it is the reason. Indeed, my smart plug has two ground holes (attached picture) and can be plugged to the main plug in 2 different manners that exchange hot leg and neutral. I tried to plug it the other way, and bingo, no buzzing noise ! If this is confirmed to solve the problem, it would mean that
1) my smart plug is cutting only one of the 2 legs
2) my Zero does not like a cutting sequence where only one leg is cut (probably if only the neutral is cut)
3) a good timer for the Zero should cut the 2 legs
@Curt : where did you see that the plug does not pass more than 800W? It is rated for 16A, i.e. around 3500W