Here is the another common error message when dealing with directory and password synchronization.
Your account is temporarily locked to prevent unauthorized use. Try again later. Contact Customer Support if the problem persists
Make sure the directory synchronization service account has proper permission in AD. Permission details are documented in this article - https://azure.microsoft.com/en-us/documentation/articles/active-directory-aadconnect-account-summary/
The above error message is related to Replicating Directory Changes and Directory Changes All permission in AD.
From https://azure.microsoft.com/en-us/documentation/articles/active-directory-aadconnect-account-summary/ “If you intend to configure password sync to Azure AD, ensure this account has the following permissions assigned: -Replicating Directory Changes -Replicating Directory Changes All”