cannot save pages posts or products which contain hyperlinks

I can’t save any changes to pages where I am adding a hyperlink or image. If it’s plain text I am allowed do it but when there’s a link in my change there’s a 404 error. Is there something that might have caused this to happen – a setting that might have got corrupted by something? Or is this a known bug with the latest version?

The page I need help with: [log in to see the link]