This is a bug with WP 3.4 – I’ve posted a ticket (http://core.trac.wordpress.org/ticket/20953)If you want to fix it right now, go into functions.php (in /wp-includes/) and on line 3625 you change
$trace = debug_backtrace( false ); to
$trace = debug_backtrace();
It will solve your problems for the time being. Also, tell your host to update the PHP-version the server is running (it is because the old version that this error occurs).
Fix From Here: http://wordpress.org/support/topic/after-upgrade-to-34-site-shows-error-code