Adding myself to the above, same issue is being encountered.
Local development site, WP 4.0, ACF PRO 5.1.1.
This only appears to happen when updating or publishing a post.
Warning: hash_equals(): Expected user_string to be a string, integer given in /...../wp-includes/pluggable.php on line 1719
Warning: hash_equals(): Expected user_string to be a string, integer given in /...../wp-includes/pluggable.php on line 1725
Warning: Cannot modify header information - headers already sent by (output started at /...../wp-includes/pluggable.php:1719) in /...../wp-admin/post.php on line 233
Warning: Cannot modify header information - headers already sent by (output started at /...../wp-includes/pluggable.php:1719) in /...../wp-includes/pluggable.php on line 1173
