The solution is quite simple but hard to find. In a sitepackage or another extension usually php-files start with the following code:
<?php defined('TYPO3_Mode') || die();
„TYPO3_Mode“ is deprecated the new correct statement is with just „TYPO3″_
<?php defined('TYPO3') || die();
The frontend in my case also produced just a white page so it was very hard to find the problem.
More information is found here: