I had setup Directory Sync from our on-premise AD to our Azure AD a while back. I made sure to set the Azure AD account that I use for sync to never expire. Recently it seems like sync stopped working because user authentication for new users was not working and there were errors in event viewer stating "Synchronization failed to start for Active Directory Connector because of credential problems. Run the Windows Azure Active Directory Sync tool Configuration Wizard again to update credentials for synchronization." I re-ran the Directory Sync configuration using the same settings and users for both the Azure and on-premise side. It all ran through fine and everything is syncing again it appears.
The only thing I can think is that the on-premise account that I used had the password changed. However, when you are running the configuration utility, it states that this credential is not saved. To me that means it is used initially and then not needed on-going. Do I have that correct?
Also, I noticed in event viewer that after I completed the Directory Sync Configuration there was an informational entry stating "Creating Domain Account (MSOL_cae84bd459e) and then a following warning entry referring to "Resetting password for domain\MSOL_cae84bd459e."