Hello,
I got it working. First problem was the internet connection via WLAN. It caused a download problem. Why? I do not know. The signal strength was optimal, all other internet applications work fine, including downloads. I connected the Pi3B to a LAN cable and now the download completed.
Then the device rebooted after klicking install. Then it said "skipping digest check, no digest file found". After that "verifying ZIP file" and then the error message "ZIP verification failed" and "error installing zip file".
So I got to the boot manger menu and pressed install, selected the zip file /data/... , unchecked "Verification of the ZIP file" and the flashed version 0.5.14. After that I did a reboot and the system was busy for approx. half an hour.
Now it works again.
Kind regards
gsfx