Error when reinstalling macOS Sierra - cannot reinstall
The previous content of the main volume has been replaced with the content of the BaseSystem.dmg in the Recovery HD by unfortunate circumstances. These circumstances are unclear. Some files have been recovered with the usual tools to an external volume.



To reinstall macOS do the following (all data on your Mac will be erased so don't proceed if you still have some data on your main volume/disk or if you want to recover it!):


Boot to Internet Recovery Boot (hold altcmdR while booting). You will see a spinning globe while booting to a NetBoot image from Apple. Depending on your Internet connection this will take some time (~5 minutes on a 50 Mbit/s line)
Open Disk Utility and erase/repartition your internal disk
Quit Disk Utility and open "Reinstall OS X..." which will install the system your Mac was delivered with.
After installing the (old) system and configuring your Mac open App Store, enter your Apple ID credentials, download Sierra and upgrade the system.