I'm using Grid for a concert page with several blog posts. (Don't think this has anything to do with my issue but thought was worth mentioning).
As far as I remember I was able to edit the posts with Cornerstone. However, it seems I can only visually or textually edit the posts now. I'm not sure what happened. My plugins and templates are all up-to-date.
This is the website: http://libertango-baden.org/konzerte/
Looking forward to your feedback.