Domain Admins cannot use RDS, but Domain Users can

We have a WS2016 running RDS.

Everyone that has access can open the web page and sign in through the https://...../RDWeb site.

Once in, ALL USERS not in an AD administrative group can then click on an icon, sign in to the second prompt and use the apps.

If I, or any admin, tries to click on an icon on the webpage, it prompts us to sign in, then it gives us an error with the three reasons you need to check for.

All admins can RDP into the same server running RDS, but none of us can go through the web page to do it.

I found something online saying to change the local GP, which we did, but that didn't work either (and yes, I forced the GP to update on the server).

Any thoughts would be helpful.

Thanks beforehand, because this is a tad annoying that no one in IT can use the services, but all regular users can.

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