So there looks like there may be an issue with editing content on the front-end. I have the login creds in the ticket at https://stackideas.com/forums/error-400-when-saving-older-articles
Steps to reproduce the issue...
1) Go to https://www.claviercompanion.com/article-details/a-legacy-of-excellence-an-interview-with-john-and-nancy-weems-1
2) Click Edit
3) Make any change to the content.
4) Click "Update Post"
5) You'll be routed to https://www.claviercompanion.com/article-details
and the article's changes didn't take effect.
Thanks again for the great support!