There's a patch on the above reference #6768 which I will try to release this weekend. I don't see how a reboot would ever fix it - it's a temporary problem caused by Facebook's web server not returning the information the documentation says it will.

Given its intermittent nature, and the timescale in which it appears, I suspect it is caused by the API not being fully up-to-date with the widely publicised changes to Facebook's privacy settings, and that is a bug in Facebook which I can work around.