I'd like to thank @lmosenk and all the devs here: the driver contained in the latest HE firmware didn't work for me, this one does, and it's also more configurable.
I spent 3 days trying to make the sensor work, but the problem was not the cabling or the sensor, it was the driver. As soon as I changed to this one, it worked immediately and flawlessly. Finally my Aeotec Siren 6 is doing its job. I completely replaced my old traditional doorbell and its old chime, with a new videodoorbell and the Aeotec Siren 6. HE is doing its magic. 
I'll tag @mike.maxwell to let him know about this, maybe he can take a look at the official driver and this one.
Thank you so much!
