BE AJAX request delivers empty repository

I want to build an BE extension that is reading the repository from another extension and delivers the data as CSV/XSL/XSLX without saving it on the server. e.g Outputs the data in a blank window with modified headers.

The BE AJAX request is fired properly with

$TYPO3_CONF_VARS['BE']['AJAX']['tx_myext::ajaxID'] = 'filename:object->method';

The repository if called from the backend does work fine too.

public function ajaxAction() {
...
$this->extRepository =& t3lib_div::makeInstance('Tx_MySecondExt_Domain_Repository_DataRepository');
...
}

but when called from domain.tld/typo3/ajax.php?ajaxID=tx_myext::ajaxID it doesnt find Tx_MySecondExt_Domain_Repository_DataRepository also if i call the repository of the second repository with the findAll() methode directly by AJAX. It does only return NULL.

also when setting the QuerySettings by hand

public function findAllexport() {
    $query = $this->createQuery();        
    $query->getQuerySettings()->setRespectStoragePage(FALSE);

    return $query->execute();
}

Also FYI its on 4.5

Edit:

Calling the repository with the ObjectManager doesn't work too

$objectManager = t3lib_div::makeInstance('Tx_Extbase_Object_ObjectManager');    
$this->extRepository = $objectManager->get('Tx_MySecondExt_Domain_Repository_DataRepository');

Did you make sure that the other extension is loaded before your own extension is loaded? Look at your localconf.php. Normally you need to specify the dependencies in your ext_emconf.php before you install your extension.

Also make sure that you have added both extension's configurations to the Static Includes of your TypoScript template.