For each of the Node that is a member of the Cluster assigned to the Azure AD Discovery, follow these steps:
1. Install the Azure AD and MSOnline PowerShell modules. For more information review the following link:
https://docs.microsoft.com/en-us/office365/enterprise/powershell/connect-to-office-365-powershell
2. Restart the Node service.
Afterward, run the Azure AD discovery again to see if the issue persists.