I meet the same issue.
I have a textarea field in editor="redactor" mode, including a <h3></h3> tag, and if I insert parentheses just after the <h3> tag, then I get the 403 Forbidden error.
I have no idea what the problem is. I tried to change the editor mode to markitup or simplemde, but it doesn't solve anything.
It is very annoying, though.
I don't have weird browser extensions and I don't think I have a virus...
Same problem happens when I want to add a <iframe> in another textarea...