Defender for Server deployed, integration for DfE checked, but M365 Defender showing "Can be onboard

StanislavBelov - Thank you for taking the time to respond, and apologies for the lag in replying. The servers have been onboarded for ~7days so the 12 hour grace period, and I would assume any other grace period should have passed.

In order to make troubleshooting steps easier and to use as a reference for others, I am going to concentrate on a single server (although all servers are failing to appear in DfE). The server in question is on-prem, running Windows 2022, with Azure Arc successfully installed. 

From the referenced article - https://docs.microsoft.com/en-us/azure/defender-for-cloud/integration-defender-for-endpoint?tabs=windows

The server meets the requirement listed under https://docs.microsoft.com/en-us/azure/defender-for-cloud/integration-defender-for-endpoint?tabs=windows#availability

To confirm that the https://docs.microsoft.com/en-us/azure/defender-for-cloud/integration-defender-for-endpoint?tabs=windows#prerequisites are met:

1. The machine(s) are connected to Azure via Azure Arc.

2. Microsoft Defender for Servers is enabled and set to plan 2

3. Allow Microsoft Defender for Endpoint to Access my data is checked

4. Defender for Endpoint Security Center shows the server as "Can be onboarded" rather than Onboarded and the associated additional information

Running the MDE Client Analyzer to confirm connectivity to required URL (no proxy in the environment) returned an error: