I ran across this issue today - and mind you it took a few days to actually identify - but it appears that when saving an article with the following characters in it you'll get a 403 error. Here's the line that creates all the problems:

<ol> <li>Kent State University, Women in Podiatry. https://www.kent.edu/cpm/news/women-podiatry#:~:text=According%20to%20the%20American%20Podiatric,cited%20in%20Delzell%2C%202016).</li> </ol>

It's just a line from a list of references, but it contained a "#:~:text=" part in the URL (so that when the page was loaded it would highlight a piece of the content on the loaded page). Nope. Apparently you can't do that. Now to be fair I didn't try doing this as a straight Joomla html code piece - I'm using the SPPageBuilder with a text element. I'm not sure why it causes the issue, but once I removed the "#:~:text=According%20to%20the%20American%20Podiatric,cited%20in%20Delzell%2C%202016)" part I didn't have any issue saving the article anymore. I'm going to guess that the "#:~:text=" part (or maybe part of it) was where it went bad.