In the latest K2 Content module you have to replace code:
$query .= " LEFT JOIN #__k2_comments comments ON comments.itemID = i.id";
with
$query .= " LEFT JOIN #__jcomments comments ON comments.object_group='com_k2' and comments.object_id = i.id";
But in my mind this modification isn't need because we have in K2 Content's module parameter called "Comments counter and anchor link".