Wagtail raises ValidationError on page update
Just faced a very similar issue.
Make sure you are not using ParentalKey in a Page model.
Use the regular Django models.ForeignKey instead.
On publishing updates to an existing wagtail page it raises a ValidationError with the message below.
ValidationError({'live_revision': ['page revision instance with id 33 does not exist.']})
All other actions such as drafting, submitting for moderation all work okay.
Only publishing the page with new updates raises the ValidationError effectively limiting the ability to update existing pages.