Just checked in version 1.2.5, which will fix (I believe) the errors you all have been seeing with 1.2.4,
This is an update everyone should apply.
After applying the update, you will need to visit the WPBook Lite settings page and use the "Generate Access Token" link once more.
In 1.2.4, I changed the way access tokens were retrieved from Facebook - to use wp_remote_request instead of file_get_contents() which is less flexible and would fail on some hosts.
In the process, I introduced a bug which was truncating the access token while storing it, rendering it invalid.
In other words, my fault.
Please do test the new version and open a new thread here in this forum if you're still seeing issues.