I recently bought a new MacBook Air (M1) with Mac OS Version 12.1 (21C52).
I downloaded the Microsoft Remote Desktop App from the App Store (Version 10.7.3 (1966)) and tried to connect to my Terminal Server at work.
The connection failed with the following error:
Unable to connect
An error occurred while communicating with the Remote Desktop Gateway. If this keeps happening, contact your network administrator for assistance.
Error code: 0x3000008
Then, I downgraded on my new Macbook to the Remote Desktop Beta Version 10.6.7 (1896) (which happens to be the version currently installed on my older MacBook) and this version is working perfectly fine without any issues.
Hence, there must be a bug in your newer Remote Desktop version 10.7.3.
Please fix it and report back to me, so that I can uninstall the older beta version.