I am having the same issues, I tried with PHP 5.4 and 5.5 but am getting the empty response when OpCache is enabled.
Interestingly I am getting this error message in my apache log:
PHP Fatal error: Cannot redeclare class lesson in /var/www/tss/mod/lesson/locallib.php on line 890
It only happens on Moodle pages that disable caching such as the admin page and individual courses.
Perhaps the information below, found at oficial PHP site (http://www.php.net/manual/en/opcache.configuration.php) may help you:
opcache.dups_fix boolean
This hack should only be enabled to work around "Cannot redeclare class" errors.
By the way, boolean means "0" or "1", so, to activate this directive in your php configuration file, you must add the line:
opcache.dups_fix=1
