Noticed this error earlier today in the Azure Portal. This happened after renaming the server over the weekend. It looks like the Agent on the server uses a certificate to authenticate with Azure. Since the server name changed, the certificate is no longer valid.
I ran the "test-azureAdConnectHealthConnectivity" cmdlet as suggested, and got these results:
It was pretty obvious what the problem/solution was after seeing the "Invalid Credentials" response from the server. I wasn't sure how to update the certificates without completely re-installing Azure AD Connect, and couldn't find anything on Google that would help. But after poking through the Azure AD Connect Powershell module, I found this cmdlet: Register-AzureAdConnectHealthSyncAgent.
After running this cmdlet, and restarting the Azure AD Connect services, the error message was resolved in the Portal. Hope this helps anyone experiencing this issue.