I am experiencing something similar but with uploading code into the editor. We have a weekly newsletter and this has never been a problem for the past 2 years. but today we see this message. not sure what we do to troubleshoot. any guidance is appreciated.
I too had the same issue yesterday while uploading release notes for our release notifications. There is a strip of code that I had to paste into the email body to advise the bug fix. I always encountered this same error, and I couldn't figure out why. Finally I figured out it was due to the one line of code in the release note! I had to edit the line of code in order for the line to stick into the email without error. without the first "/" before the line of code 🙂 I hope this helps. I never encountered this issue previously either.
Can you please be more precise about the change you made so future readers can learn? Provide before-and-after examples of the email content.