RE: Web Client GP 2016 R2 hangs
Normally when Web Client hangs here, it is having an issue finding the Runtime Service, which is what is used to load the GP application into the browser window.
You mentioned you're using a scale-out deployment, how many servers do you have in this environment for Web Client?
Also, what type of certificate are you using?
Are you using tenant services at all?
What we commonly see is that a third-party and/or wildcard certificate will be setup for use with Web Client, for example using a host name such as ESPN.com, which isn't the FQDN at all. What happens then, is that when we tie the certificate to the Runtime Service, Web Client will look for this service via the host name applied to it and because of that, there needs to be something that will let the servers know that when they receive a call for ESPN.com, that it needs to be routed to the IP address of the session service server, where the runtime service will exist.
A good test of this is to ping the host name setup on the Runtime Service from all machines users are attempting to access Web Client from, and verify that it returns the IP address of the session host server where runtime services exists on. If it doesn't, you'll need to setup some type of forwarding to go from the host name to the IP address of the correct server.
The 'Dynamics' log in the Event Viewer on the server may also have additional information that can help with this type of troubleshooting.
Thanks