RE: Web Client Remote Access Issue
Being that we bind the certificate Web Client is using, in IIS, to the site that you're installing Web Client onto for HTTPS and, by default, port 443, as well as tie the certificate to the Runtime Service for Web Client, which is used to load the application into the browser window, I'm not sure we've ever seen anyone try to publish the IP address so the URL changes adding a different port number then what is actually setup during the installation of Web Client.
My thinking is that you'd need to have some type of DNS setup or forwarding so that when a user enters this https://IP:5080/GP URL, it knows where to go to find the server that Web Client is installed onto and to find the services Web Client uses: Session Central Service, Session Service and the Runtime Service.
I would also recommend looking in the Event Viewer on the server, under the Dynamics log, as that is where Web Client will write all of its error and warning information and most likely you'll find more information related to this error message you're seeing in the browser window.
If the client wants Web Client to use https://IP:5080/GP, I'm not sure if they may not have to reinstall Web Client to use that URL and port number. Definitely nothing I've seen tested nor tried yet.
Thanks