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.