[TYPO3-mvc] Class not autoloaded

[Jonas Götze]

Hi,

I use TYPO3 7.6 and I am trying to overwrite a controller class using

config.tx_extbase{
	objects {
		Some\Ext\Controller\FooController.className = My\OwnExt\Controller\BarController
	}
}
but I always get this error:
Oops, an error occurred! Code: 20151115132924881d7a1e - {"exception":"exception 'TYPO3\\CMS\\Extbase\\Object\\Container\\Exception\\UnknownObjectException' with message 'Could not analyse class:My\\OwnExt\\Controller\\BarController maybe not loaded or no autoloader?
 in [...]typo3_src-7.6.0\/typo3\/sysext\/extbase\/Classes\/Object\/Container\/ClassInfoFactory.php:37
I have double checked all names and cannot see any fault.

Are there any changes in 7.x I have overseen?
I also tried https://docs.typo3.org/typo3cms/extensions/core/latest/Changelog/7.5/Feature-68700-AutoloadDefinitionCanBeProvidedInExt_emconfphp.html but without any changes.