The cause of the issue is a certificate not being installed for the Terminal Services gateway. The Terminal Services gateway is used by RWW. The fix is to select a valid certificate and then all is good.
To fix the issue:
- Open TS Gateway Manager from Administrative Tools.
- In the left pane, right-click the server and click Properties.
- On the SSL Certificate tab, click Select an existing certificate for SSL encryption and click Browse Certificates.
- Select a valid certificate and click Install.
- Click OK to close the properties of the server.