@rexi yep. It looks like the client software assumes that there is a proxy or something running locally for Tor: tails-upgrade-frontend-wrapper stops with Error while downloading the signing key ... Could not download 'https: //tails.boum.org/tails-signing-minimal.key', request failed (Couldn't connect to server): Failed to connect to 127.0.0.1 port 9062: Connection refused...
Now the UI is slowly downloading the upgrade through Tor :)