OPcaching creates PHP errors

On our server running several instances of Moodle 2.7.2 under Apache 2.4.6 and MariaDB 5.5.33. Now we changed our system from PHP 5.4.20 with APC 3.1.13 to PHP 5.6.2 without APC using the internal OPcache like recommended here https://docs.moodle.org/27/en/OPcache. The OPcache uses the required settings for Moodle - only the values for opcache.memory_consumption and opcache.max_accelerated_files are much higher because of the multiple instances.

Since then we have A LOT of notices, warnings and errors in the Apache error logfile e.g.:
PHP Notice: Constant NO_DEBUG_DISPLAY already defined in /MOODLE/www/htdocs/XXX/lib/javascript.php on line 27
PHP Notice: Constant ABORT_AFTER_CONFIG already defined in /MOODLE/www/htdocs/XXX/lib/javascript.php on line 30
PHP Warning: require(../config.php): failed to open stream: No such file or directory in /MOODLE/www/htdocs/XXX/theme/jquery.php on line 34
PHP Warning: require(../config.php): failed to open stream: No such file or directory in /MOODLE/www/htdocs/XXX/theme/image.php on line 32
PHP Fatal error: require(): Failed opening required '../config.php' (include_path='.:/usr/share/php5:/usr/share/php5/PEAR') in /MOODLE/www/htdocs/XXX/theme/image.php on line 32
PHP Fatal error: require(): Failed opening required '../config.php' (include_path='.:/usr/share/php5:/usr/share/php5/PEAR') in MOODLE/www/htdocs/XXX/theme/styles.php on line 30

The XXX vary through all Moodle root folders of our instances.