Hi David Hancock,
Could you please check:
Try removing and re-adding the workspace/feed URL.
Try signing out and back into the Remote Desktop client and Verify MFA is working properly.
Update or Reinstall the Remote Desktop Client: https://support.microsoft.com/en-us/windows/how-to-use-remote-desktop-5fe128d5-8fb1-7a23-3b8a-41e636865e8c
If you have any further queries, let me know. If the information is helpful, please click on Upvote and Accept Answer on it.