Steps to reproduce:
1. Create any site and invite any user as a SiteCollaborator;
2. Create any post as non-SiteCollaborator user;
3. Log in as SiteCollaborator and click "Edit" link for post created by other user;
4. Make changes and click "Update" or "Update Internally and Publish Externally" button -> the next notification appears:
Unable to save the blog post
SiteCollaborator is able to edit posts created by other user.