With ModSecurity installed on the server, if you go to edit any page (not homepage) and then click for example the homepage on the breadcrumb links, a 403 error will be triggered because the breadcrumb urls ending with ?open=[page ID you're clicking] to keep that parent's children pages opened on the lister.
The "open" must be renamed to something like "pages_open" to prevent false positive triggers.
I don't know if this "open" query is used on other admin areas but it should be definitely renamed everywhere.