Hey ! I had the same error when updated to 1.5.7. I used luigivieira solution and it worked but now I have this message under titles of my post :
WARNING: CALL_USER_FUNC_ARRAY() [FUNCTION.CALL-USER-FUNC-ARRAY]: FIRST ARGUMENT IS EXPECTED TO BE A VALID CALLBACK, 'RETURN ' ITEMPROP="DISCUSSIONURL"';' WAS GIVEN IN /HOME/PASTOUTD/WWW/WP-INCLUDES/PLUGIN.PHP ON LINE 213
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