I think you know how the rest of that error message goes.
We are hosting on WPengine and have a WP Network site using individual X licenses on 5 - 6 sites. All other plugins upgraded ok. We are running latest version of X (At least it dosent say we need to update it). But when upgradering Cornerstone and going to edit a page on any of the network sites, this error pops up.
The only fix is to roll back to a previous backup and work with the old Cornerstone.
We have no idea how to fix this.
After posting, I found this thread and is following its lead:
Thomas