There is debug output on the WordPress login page from PowerPress. First, why is powerpress even trying to do anything on the login page??
Second this can easily be fixed by changing powerpress.php line 138 to the code below.
else if( is_object( $post ) && current_filter() == 'the_content' && $g_powerpress_excerpt_post_id == $post->ID )
Not the check for an object before we try and run anything.