I've noticed an additional clue that contains this issue.
I tried recreating my homepage to see if I could bypass the original cornerstone issue- but I noticed when I was populating my page with POST forms, cornerstone would no longer load the content which would not let it save the web page.
In this specific example, this snippet of html was preventing cornerstone from loading/saving:
<form action="//" method="post" id="mc-embedded-subscribe-form" name="mc-embedded-subscribe-form" class="validate" target="_blank" novalidate>
The moment that snippet was removed, the mailchimp form loaded into the text box and was able to save, of course, now it cannot perform it's action. Hopefully this helps the support team gain more insight into what's going on with this cornerstone fiasco that's affecting lots of users.
P.S. I am still receiving the issue:
Failed to load resource: the server responded with a status of 406 (Not Acceptable)