At some point during the 1.4.1->1.4.3 series, my test copy of WPBook Lite was out of synch with the repository – so users were experiencing bugs I was not able to reproduce.
This affected potentially all users who were using WPBook Lite with long-lived access tokens – WPBook was not storing the correct length of access token.
If you were affected by this bug, you would constantly be resetting your access token and although WPBook Lite would say it succeeded, the resulting token stored was faulty – too short. If you testing that token using the Facebook Debugger it would say failed to decrypt.
Thanks to users in the forum who shared debugging info that lead me to find the right issue in the code!