An article with the characters "/=" (without quotes) in the article text, generates error "Forbidden You don't have permission to access this resource", regardless of which editor you use.
How can I prevent this?
This sounds like a server side process that is generating a 403 (Forbidden) error based on the content. If you don't have any "security" extensions installed that might also be the cause, you will need to speak to your hosting company about this.
Just because you're not paranoid doesn't mean everybody isn't out to get you.