I noticed the following error on that page where you define connection for User Profile Service Synchronization.
An error has occurred while accessing the SQL Server database or the SharePoint Server Search service. If this is the first time you have seen this message, try again later. If this problem persists, contact your administrator.
It was working fine before and now all of a sudden this error. Because of it, UPS is not syncing with AD. I did
iisreset /noforce on APP server where CA is hosted but it didn't solve the problem. What to do?