The address I have sent you is to make sure that the SRTM 3Sec data is correctly downloaded, because this has changed recently. So you don’t access the page to download it yourself.
As described here, you go to your snap installation directory and find the file called “snap.auxdata.properties” inside the ect folder
Open this file with a text editor and go to line
and replace it by
Save the file and close it. Next time SNAP searches for SRTM 3Sec data, it will access this source instead of the old one. The BackGeocoding step should then work again.
This will be fixed with the next update, but if you want to have SRTM 3Sec data automatically downloaded before you can change it yourself as described above.
Another thing is to make sure that SNAP is generally allowed to access online resources. Please make sure that port 443 is open in your firewall settings. How to Open a Port on Windows Firewall
Windows and Mac: How to open ports in your firewall