Hello Barno,
I had the same problem and I think I found it:
The configuration.json file is in your installation directory in the folder configuration. It's just another name : orthanc.json
C:\program files\orthanc server\Configuration\orthanc.json
The file is automatically create in the installation.
Change inside the orthanc.json :
"RemoteAccessAllowed" to "true"
and after you can access to the page:
I just discover in the same moment. So funny :) Hope this information can help you.
Best regards,
Bruno