I am having the same problem. same functions environment.
Warning:Parameter 1 to Abovethefold_Optimization::end_buffering() expected to be a reference, value given in wp-includes/functions.php on line 3464.
and on that line I have this bit
<blockquote>
function wp_ob_end_flush_all() {
$levels = ob_get_level();
for ($i=0; $i<$levels; $i++)
ob_end_flush(); (this is line 3464)
</blockquote>