Error validating access token: Session has expired

And version 2.8 of the plugin. I have started getting this:

Error validating access token: Session has expired at unix time 1361580022. The current unix time is 1363875031

…every time I try to post. Social will still post to Twitter, but posts to Facebook fail.

Is this a known bug or do I have to go searching? If it the latter, any suggestions as to which module is the culprit? Caching? I’m using https://www.w3-edge.com/wordpress-plugins/w3-total-cache/ for example.

https://www.ads-software.com/extend/plugins/social/