1) disable ALL pluging and test again - could be a plugin that is not 4.0 compatible
if that doesn't help
2) edit wo-config.php and add
define('WP_DEBUG', true);
define('WP_DEBUG_LOG', true);this will log any messages to a file 'wp_debup.log' in the wp-contents' folder. Test and check the log file. Also look at the server's error log (you may have to contact your host to find this) and see if there is any information.