Getting "This page does not contain any regions that are editable in safe mode" on ALL non-master pages
<p>In SharePoint Designer 2013, I'm getting the following error when attempting to edit <em>any page</em> on the site (other than a master page):</p>

<p>"This page does not contain any regions that are editable in safe mode. Do you want to open this page in advanced mode?"</p>

<p>This includes pages with WebPartZones; even those regions are completely blocked off from editing. As an example, I created a blank Web Part page in SPD, and this is the result immediately after opening it (and declining to use advanced mode):</p>

<p><a href="https://i.stack.imgur.com/id7zS.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/id7zS.png" alt="A brand new Web Part page with a non-editable WebPartZone"></a></p>

<p>I'm stumped on this one.</p>