Hello Thomas Gellar
Welcome to Microsoft Q&A Platform, thanks for posting your query here.
Based on the issue description, the issue started after upgrading to MacOS Sonoma.
Are you able to connect to the Windows 2019 Datacenter at all, or does the disconnection happen immediately after connecting? Also, have you tried connecting to the Windows 2019 Datacenter from another device to see if the issue persists?
Assuming that you are able to connect to the Windows 2019 Datacenter, but the disconnection happens after a period of time, there are a few things that you can try to resolve the issue:
-Make sure that your Mac mini is connected to a stable network connection. If you're using Wi-Fi, try connecting to a wired network to see if the issue persists.
-Open the Remote Desktop app on your Mac mini, click on the "Session Options" button, and then click on the "Experience" tab. Try adjusting the settings to see if it makes a difference.
-Open the Remote Desktop app on your Mac mini, click on the "Session Options" button, and then click on the "Advanced" tab. Under the "Display" section, try disabling "Hardware acceleration" to see if it makes a difference.
-Open the Remote Desktop app on your Mac mini, click on the "Session Options" button, and then click on the "Advanced" tab. Under the "Connect from anywhere" section, click on the "Clear Cache" button to clear the Remote Desktop cache.
If none of the above steps resolve the issue, you may want to try uninstalling and reinstalling the Remote Desktop app on your Mac mini.
Hope this helps.