Spontaneous page>update failure

Running latest version of WP with Constructor theme. I’m suddenly unable to update pages following months of no difficulty whatsoever. I can access dashboard, access page edit, edit type in html; but, when I click Update, I’m sent to https://vermontgreenoffices.com/wp-admin/post.php with message, “Error 404 – Not Found… Sorry, but you are looking for something that isn’t here.”