MacOS Sierra not recognizing external displays and going black

Hi.

I've just upgraded to macOS Sierra on my MacBook Pro Retina Mid 2012.

After the upgrade, my Mac is not recognizing any external display:

I've tried HDMI and DisplayPort to VGA adapters.

The Retina monitor "blinks" but not signal is detected on external displays.

Also, after rebooting, also the retina monitor goes completely black (the mac has booted, because I can trigger Siri commands!).

To restore the situation I need to put the mac to sleep, plug in an hdmi monitor (which does nothing), wake up the mac and eject the plug. After this sequence the retina monitor goes back on (and only that!).

I've tried to reset NVRAM and SMC: no effect.

I've tried to boot into safe mode or recovery mode, but the retina monitor remains black, as well as any other external display (the same happens if I try too boot with or without a connected external display).

I'm stuck and have no idea on what to do. I cannot even try to reinstall the OS, as no video works just after reboot.

Any hint would be appreciated.

Thanks!