I just upgraded from 3.3.13 to 3.4.1, which goes correctly now (had some issues, solved now). I read somewhere that enabling the container.dumper.inline_class_loader could dramatically speed up development environments, which I can confirm. Load times for the first page tested on development was 50% faster.

The first request works after clearing cache, but my second request (doesn't matter which page in my system) runs into the following error. Without the parameter enabled, this error does not occur.

FatalErrorException:
Compile Error: Access level to Symfony\Cmf\Bundle\MediaBundle\Adapter\ElFinder\PhpcrDriver::$options must be protected (as in class FM\ElFinderPHP\Driver\ElFinderVolumeDriver) or weaker

class PhpcrDriver extends ElFinderVolumeDriver

$options in PhpcrDriver is private. In ElFinderVolumeDriver it is indeed protected. I understand that this is an issue anyway.

But I'm not sure why it only errors out when this parameter is enabled and not before..