i have latest version forum, Invision Community v4.7.13

when adding longer post with pictures etc, cant add post, after edit or post pages goes white, after checking post, edit or new post not added

testing from diffrent pc pages goes 403

Would advise working with your hosting provider or server administrator to see if your WAF or mod_security blocking these. This is not a software error, per say, happening here but rather you're exceeding a limit on your WAF or server causing the request to be blocked.