Fixed it. I waited 1 hour with the circuit breaker of the cam off, but it still didn't work. So I phisically removed it from the wall, unplugged the power cables, etc. brought it in the house, connected it with my test 12V AC transformer and it finally powered on correctly.
I have no idea why simply turning off the circuit breaker for 1 hour is not enough. In any case, I reset it to factory settings with latest Hikvision's fw, and reconfigured it as an ONVIF device with Synology: more options are available with the Surveillance Station software and the ONVIF interface. So now I'm back on track but with some more tweaks.
I will test the Hikvision-Connect app a little bit more, but it looks very much similar to Ezviz, with the added bonus that I can configure the chime in this, with the latest Ezviz app the option was not there anymore.
@David L : remember when I replaced the cam in warranty and the new one could only be accessed with the broken cam's 6-digit enabler code? We thought it could be due to the fact that I restored the configuration on the replacement cam. But that's not the case: after the upgrade from ezviz fw to hikvision fw, I factory reset the cam, reconfigured it from scratch, and it still works only with the original cam's enabler code. I think the code is tied to the serial number of the cam in their cloud, and it is associated with the user account.