The error "Error saving plugin settings: Could not register story template with Facebook: Unknown method" has been popping up more and more among users of the FacebookCommenters plugin for Movable Type. The cause is a change to the API at Facebook. Six Apart is aware of the issue (check the Fogbugz ticket here if you have access) and a fix has been committed but not released yet. I have been told the updated version of FacebookCommenters will be bundled with the upcoming Movable Type 5.13/4.38 release.