Some user have frequent http403 forbidden error message on my forum. Some have no problems. Any ideas? I attached a sreenshot
http://wiki.simplemachines.org/smf/Mod_security_-_Having_problems_with_mod_security
Go through the server logs (if you have access to them) and find all occurences, timestamps etc of the 403's, then have a serious chat with your host.
& check content of .htaccess (if in use) and see if anything there may cause trouble. For example, have seen .htaccess 'deny' 403's spilling over to legitimate ip's due to server slowness\caching...