Here is what it says:
Challenge Not yet complete... here's what's wrong: There was an unexpected error in your org which is preventing this assessment check from completing: System.CalloutException: IO Exception: External server did not return any content
Tried on different orgs, but without success. Even tried from scratch with the guide http://ashishkeshari.com/index.php/2017/03/01/superbadge-data-integration-specialist/
Any suggestions?