If you've recently changed the password for a specific ServiceBox username, and try to engage the accounting connector, you may experience this message:
When this happens it's because your user password was hardcoded into the accounting connector at the time of set-up, so if that password is changed, it also needs to be separately changed in the accounting connector.
Just open the accounting connector and head into the set up (upper right). Change the password in the ServiceBox area, and then test the connection. Once it succeeds you can hit ok at the bottom and start your sync.