Can not post image / file / video on wall. It says "Forbidden - You don't have permission to access /activity/post/ on this server" ?

Solution :

You need to disable PHP Mod Security on your server via .htaccess

Open .htaccess file and insert this code

<IfModule mod_security.c>

SecFilterEngine Off

SecFilterScanPOST Off

</IfModule>

Save htaccess. But that code doesnt work to some servers. So you need contact your hosting provider support to disable it.