WordPress RPM not showing plugins update

We have a WordPress multisite 4.2.2 RPM installation on CentOS 6.6 and in the network plugins panel, the add plugin button and the plugins updates are not showing up (100% sure some plugins are not up to date).

The odd thing is when heading to /wp-admin/network/plugin-install.php, it says we haven’t sufficient permissions to access the page. Note that we have a superadmin account.

So far, we tried to comment the config line define( 'DISALLOW_FILE_MODS', true ); and changing all the salt in the config … no luck.

We tested our server connection to api.wordpress.org:443 and planet.wordpress.org:443 without any problem.

Also note that we have a second WP installation on the same server (not multisite) which is working as intended.

Any clue is welcome, thanks !
SO : stackoverflow.com/questions/31006677/wordpress-rpm-not-showing-plugins-update