Why am I seeing "access revoked" or "authentication failed"?
This is always due to the once connected app now denying access to PieSync, because of any number of reasons:
Specifically, changes to the account credentials (e.g. changing the password) may be a deliberate safety measure, but naturally, this will prevent PieSync from accessing the contact data any further. Therefore, you need to re-authorize your app for PieSync to be able to read & write to it.
You have the ability to reauthenticate your connection using the red banner when accessing your configuration page.
If you have further questions, feel free to contact firstname.lastname@example.org.