It's not showing the error I expected to see, though it's very related to it.
There's a large possibility that the script was sending a POST request, returned a 403 and WordPress failed to handle the failure (looks like there's only .success()).
Getting right to the point, mod_security is responsible and you seem to be triggering an active rule. This can be to do with what you're posting.
Before you press 'update,' what changes have you made? If you edited a post, did you by any chance add any URLs or code (HTML, JavaScript)?