if the auto update does not work, please perform a manual upgrade instead. Please download the installer from your customer account and just run the same on the machine where PRTG is installed. All your data will be kept during this process.
The auto-update function should work in V12 again.
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.