"Forbidden" Error when saving a particular page

I get a “Forbidden You don’t have permission to access this resource.” error when saving one particular page. Other pages seem to be fine.

There is a workaround that may inform as to why I get the error:

If I delete the entire text (from text view) from the Mobile Editor and update the page it saves as normal. Then I can come back in and copy and paste the code (from text view) from the regular editor to the Mobile Editor and update and it saves as normal.

No shortcodes. Just text and links.