Hi Claudio,
I have the same problem.
Today I called with the sys admin of my server and he said me that he found this errors in the log
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[03-Mar-2016 17:47:16 UTC] PHP Fatal error: Call to a member function add() on a non-object in /home/public_html/xxxxx.com.ar/wp-content/plugins/woocommerce-mercadopago/includes/class-wc-mercadopago-gateway.php on line 487
[03-Mar-2016 17:48:27 UTC] PHP Fatal error: Call to a member function add() on a non-object in /home/public_html/xxxxxx.com.ar/wp-content/plugins/woocommerce-mercadopago/includes/class-wc-mercadopago-gateway.php on line 487
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1) I dont have iTheme Security plugin
2) Mod_security is ok (or it seems to be OK)
3) I dont use CloudFare
Thanks !!