I get this error on pages where I have put in a long custom permalink for a page or post.
My workaround:
- Remove the custom permalink via WordPress’ standard Editor (copying it for safekeeping later)
- Save the page with the auto-generated permalink
- Edit via Cornerstone as normal
- Back on standard Editor, add back in the custom permalink and save
Of course there’s no guarantee for everyone, but it worked great for me.