If you are receiving the below error during eSync check to see if TLS 1.0 is disabled on the server.
The caller was not authenticated by the service. Error executing strategy 'OnAfterESyncRunErrorFromPeer' remotely. Type 'System.ServiceModel.Channels.ReceivedFault' in Assembly 'System.ServiceModel, Version=126.96.36.199, Culture=neutral' is not marked as serializable.
TLS 1.0 is required for eSync to function in versions 9.1SP2(including the latest CU for 9.1SP2) or less.