Hi Jaime,
Many sites automatically redirect you to HTTPS even you type http://… (FB, Twitter, Google, YouTube, …).
The embedded plugins will redirect you also to HTTPS.
If you need as some users to use/access these sites which have embedded plugins, you must to create exceptions as is written here (I have not tried but should work):
http://docs.nethserver.org/en/latest/web_proxy.html#bypass
BR,
Gabriel
PS
Maybe is usefull:
Profile for web content filter
or:
http://wiki.nethserver.org/doku.php?id=howto:how_to_configure_web_proxy_in_transparent_with_ssl_mode