The Problem was, that the AjaxDispatcher from pt_extbase in TYPO3 6.1 needs the parameter "vendorName" to work. i've modified the AjaxDispatcher class and then it works.
I've added the one line to the buildRequest function:
protected function buildRequest() {
/*... some Code ...*/
$request->setControllerVendorName($controllerName);
/*... some Code ...*/
}
$controllerName was set before. I read the Value from a get-parameter.