Hi Barbara,
The problem arises from a very outdated translation of the 'strfdateformatcsv' string in questionnaire German lang file.
It was translated as $string['strfdateformatcsv'] = 't.m.j h:m:s'; but it should be $string['strfdateformatcsv'] = 'd/m/Y H:i:s';
I have now fixed this in the AMOS translation and, once it is approved by the German language pack maintainer, it will be available if you update your Moodle German language pack.
However, if you want to do it urgently yourself, all you have to do is edit your moodledata/lang/de/questionnaire.php file and make the change yourself. Don't forget to empty all your moodle caches after that.
Joseph
PS.- Barbara, if you (or any German speaker using Questionnaire) want to contribute to the German translation of Questionnaire, there are currently 79 missing strings in version for Moodle 2.5!