[TYPO3-mvc] Class ActionController not found

[Philipp]

You should learn about namespaces... a namespace is not a path to a directory, it can -and should- very much the same most times will not be....

In TYPO3 a namespace is build from a vendor name and the extension name, this is calculated by the configured Vendorname and the extension key.

If you register your plugins like VendorName.[something] extbase will use the namespaced schema, if not extbase will use the underscored schema. If your extension key is test_extension the namespaced schema is

VendorName\TestExtension\Controller\TestController

The underscored would be

Tx_TestExtension_Controller_TestController

The files will allways be in the 

EXT:/extension_key/Classes Folder

System Extension have the Vendorname TYPO3\Core

System Extensions can be found in typo3/sysext/

In every extension the home directory will be the Classes Folder, you wont see it in the Classname or Namespace, so the Class of Your Controller \VendorName\TestExtension\Controller\TestController will be found in the directory typo3conf/ext/test_extension/Classes/Controller/TestController

A Class \VendorName\TestExtension\Domain\Service\SomeDomainSpecificService would be found in ...Classes/Domain/Service/SomeDomainSpecificService.php. If you would nout use namespaces the Classname needs to be Tx_TextExtension_Domain_Service_SomeDomainSpecificService

Read some articles about PHP namespaces.