I've investigated but I could not trace what's causing issue because it's to the database which we don't have access and control over Some data might have been corrupted.
The issue is not with Pro because your page couldn't really be saved even just changing the page template in the WordPress editor.
I've saved your page as a template and loaded it in a test page and it's saving just fine (see secure note) so nothing in the content is causing the issue.
The solution, though I understand would require work, would be to save the corrupted pages as a template and then delete them. You will then create a new page for each and load the corresponding templates. In case you're not aware yet, please see the Template Manager documentation to learn how to save your page as a template.