iCloud signin "Failure to verify your identity"

A client of mine is trying to login into her iCloud account via a browser on a remote desktop Windows 10 machine. Meaning she is connected via RDP to a Windows 10 machine and is then trying to login to iCloud using Firefox, or Edge, or Chrome, and every time it gives her the message "Failed to verify your identity, Try again". I'm not sure if she has 2FA enabled, but even if so, it never gets to the point of sending her a code as this error comes first.

When it says "Verify your identity", is there something more it's looking at other than the password? Normally whenever I sign into my apple account from a new machine, it still accepts my password, but just sends me a 2FA code after that initial step.

We recently changed hosting providers for our remote desktop. It used to be running Windows Server 2008 R2 and the machine was completely isolated from the public internet, but she was able to use iCloud on that machine. The new machine is a Windows 10 pro system and is also completely isolated except for the RDP port. But since she's connecting through a browser, I doubt it would be a port issue since the browser couldn't open ports to listen on plus it worked with the old RDP machine.

Both hosting services are located in Los Angeles where we are.

Any thoughts?