This article will be very short and focus one of troubleshooting I faced over the last week when I should have re-installed the AD connect for certain reason.
over the installing, as you know the wizard, I’ve provided the Global Administrator and on-premises Domain Administrator user, post these wizards I got the following errors:
Azure Service Connectivity failed, Unable to Procced: The on-premises synchronization service is not able to connect to Azure Active Directory, Updating the proxy settings for thr ADsync account may reolsve this issue. Azure End Point HTTPS://LOGIN.WINDOWS.NET/PELEGIT.CO.IL ADSync Service Account Pelegit\AAD_38424d15cfd9 Extended message: Value can't be null Paramenter name: DomainOrPrincopleName.
The issue was that this user: AAD_38424d15cfd9 hasn’t UPN in the on-premises AD:
As soon as you have set it, close the wizard and re-run it from the beginning, and then it will work.