Hey Bongobong,
Yes, the problem appeared related to Swift Frameworks interaction with (…or dependency upon) the generic WordPress element TinyMCE. Specifically, I removed the WP Admin user who originally installed the theme (and therefore, the Swift Framework), because that person was no longer part of the company. As the new Admin, I inherited the website and the WordPress installation. I had no way to determine the valid license continuity from the very first, original WordPress installation through the current Dante (we’re running Dante’s Child, to be specific) install. One of those tiny keys associated with that particular Admin user no longer matched my new Admin account, so (what I believe was) a licensing check failed, and the code persistently returned a 403.
Solution? Yeah, I wasn’t excited about this, either; but it worked perfectly and immediately! I performed a domain rebuild, fresh WP 4.x install, fresh Dante (and Child) install from a virgin .zip, and I DID NOT import the database, to avoid re-introducing and old key, an old user, or an old hash somewhere that would become a problem down the road.
BAM! Just like that: problem gone, and the site immediately began responding more quickly, too. I sure hope this helps someone. I spent a good piece of 45 days on it. ๐