SmoulBuchholz Probably not enabled.
OPcache is >PHP5.5 caching solution:
OPCache improves PHP performance by storing precompiled script bytecode in shared memory, thereby removing the need for PHP to load and parse scripts on each request.
You can clear the cache by creating clear_cache.php and with below code included and navigating to it, it should show a blank page.
Worth the try but not sure if it will resolve the issue.
Other option is to enable debug in config.php:
'debug' => false,
'debug' => true,
I am not very familiar with the output but I am sure someone here can help you trace the cause