Hi @Pete Y
Thank you for reaching out to the Microsoft Q&A platform.
Based on the information you provided, it appears that the old Remote Desktop client is not working, but the Microsoft Remote Desktop Beta client is working fine.
It is possible that the issue is related to a bug or compatibility issue with the old Remote Desktop client. I would recommend checking if there are any updates available for the old Remote Desktop client and installing them. The latest client version is "10.9.2".
If this does answer your question, please accept it as the answer as a token of appreciation.