Hi there. I had this exact same error. When I deactivated the Headspace2 plugin I could edit the page. When I re-activated this plugin the error returned. I then did a clean-up of all the old revisions of my pages with the following SQL query:
DELETE a, b, c
FROM
wp_posts a
LEFT JOIN
wp_term_relationships b
ON (a.ID = b.object_id)
LEFT JOIN
wp_postmeta c
ON (a.ID = c.post_id)
WHERE
a.post_type = ‘revision’;
This fixed the problem! I hopes this helps …