A couple of suggestions:
1. please check if the service is started (using ArcGIS Manager) and can be previewed.
2. Clear REST cache (http://servername:8399/arcgis/rest/admin or the rest/admin endpoint where REST is deployed)
I'm also not able to log into the REST Admin even though my user name is part of the agsadmin group, so I can't clear the REST cache either. I suspect that if I can clear the REST cache then I might be good to go.
I can't log on the the rest admin console with any account, be it local in the agsadmin group or the network admin account.