Unable to post comments

Updated my website few days ago.
When the plugin is enabled, I can’t post comments and the “Like” Button disappear from all the pages of the website.
And now, when the plugin is disabled I can post new comments but the “like” button don’t appears in all the webpages.
I tried to disable & re-enable all plugins, check all configurations, ecc ecc
Any idea? Can you suggest me what further can I check?
Thank you

http://wordpress.org/plugins/facebook/

More informations
“Facebook Social Plugin v.1.5.4”
Wordpress 3.8