Encountered "the object does not exist" Error When Clicked on the "Populate Container" button

I had a User Profile Synchronization (UPS) setup 2 years ago in SP2010 to sync with Sun LDAP server. Recently, I tried to edit the connection. When I clicked on the "Populate Container" button, I received an "the object does not exist" error. Anyone knows what had probably went wrong? The LDAP server is still alive and kicking, but could it be my LDAP account had its permission restricted?

Thanks!