Error 404 simply means that there is no content in the link you've clicked.
You would usually encounter this message after clicking on the Pencil icon. To fix this, simply delete the page affected by this and create a new one. The editor should work on any other page. No need to worry about lost content because the page contains nothing in it in the first place.
If you're a Contributor and your page is affected by this error, kindly send us a request and we'll fix the page for you.