403 Forbidden Error when submitting update page

I'm stumped and could use some direction. This site is several years old and this update page has worked fine in the past. All of the sudden, I get a 403 Forbidden Error upon submission of the form. I did add a field recently, so I tried reverting to the previous version of the page to see if my changes entered an error, but the previous version gives the same error. A few other things tried:

- The page is located within a password protected directory. I removed the password protection temporarily to see if that was causing an issue. No dice.
- Permissions for that folder are set to 755. I temporarily set to 777. Still no dice.
- I have numerous other "update" pages that were generated at the same time this page was and they still work without error.

I've attached the page, but I'm at a loss as how to trouble shoot further. Any direction would be appreciated.

Ray? Anyone have any thoughts here?

It could be a security module installed preventing html code from being posted... I see you are using an html editor and sometimes security is added to a site to prevent html injection that basically prevents html editors from working. I'd ask your hosting provider to see if that is what is going on.

A good test would be to make the editor field blank when submitting and see if the error goes away. That would be a sign that the html in the post is causing the error.

So I did some more testing to see if I could further isolate it. First, on your suggestion:
- Removing everything from the HTML editor field gives the same error
- On the insert page, uploading HTML via the editor field works without incident
- I noted the record they were trying to edit when they first reported the error has external links in the HTML field, so I removed those, same error
- We also have field with an email address, tried dropping that, same error
- We also have a field with a map embed using iframe. Tried dropping that, same error

HOWEVER, we have fields for social media links and whenever we have the URL string that contains "HTTP://" or "HTTPS://" in one of those fields it throws the error. If we don't have that in, it updates fine.

I'm following up with the host.