Domain controller showing "Not ready for migration" in Defender for Identity

Question

I want to get the MDI sensor upgraded to v3 on our domain controllers. When I go into Settings > Sensors, two of our DCs are listed as "Ready for migration", but the third says "Not ready for migration".

As far as I can tell, this DC meets all the prerequisites listed at Migrate from sensor v2.x to sensor v3.x - Microsoft Defender for Identity | Microsoft Learn.  In the Troubleshooting section of that article, it says "hover over the status on the Sensors page to see a tooltip that lists the reasons the server doesn't meet the migration prerequisites", but I see no such tooltip, no matter where I hover.

I tried opening a support request with Microsoft 365 Support, only to be told that I have to contact Azure support. We don't currently have an Azure support plan, so I guess we don't qualify for support.

Anyone got any suggestions for things I can try?

Update: I guess the Microsoft Support technician was mistaken when they told me that I had to contact Azure support, because when I followed up to ask for clarification, I got contacted by a different support technician who is helping me.