Why am I seeing "Authentication failed"?

In this case, the connected app is denying access to PieSync, because of any number of reasons:
- A time-out after a prolonged period of inactivity
- The account's password was changed
- The account no longer exists, or had its changed;
- The account no longer fits the required conditions, e.g. as a consequence of a billing issue;
- ... or somewhere along those lines.
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.
- The PieSync connections tab provides an overview of your connections. The ones that need attention will be presented on top.
- A dropdown list will appear after clicking the connection box, allowing you to reauthenticate the account.
If this is not working, try to remove the connection completely and set up a new one.