Hi there,
Thank you for the information. I checked the case and found out there is an internal server error 500 when the builder tries to get the cornerstone-endpoint:

After further investigation and adding the cURL with the cs_debug=1 parameter which enabled the debug mode I get this error:
curl: (52) Empty reply from server
I did a google search for this error and most of the articles indicate that there must be a Firewall or Proxy blocking the way to get a response from your HTTP server.
Please contact your hosting service provider and give them the information I shared with you and ask them to check things out on the server. Ask them to check their error log as they might have better understanding of the problem.
Thank you.