Get rid of absolut links / redirects
Finally figured it out:
Issue derived from the language negotiation settings.
Under: admin/settings/language/configure Domain name only was configured and I switched it in the database (manually updated the variables table):  

UPDATE `dinilu6`.`variable` SET `value` = 's:1:"2";' WHERE `variable`.`name` = 'language_negotiation';


Afterwards I needed to clear the cach with drush cc and then it finally worked.