First of all, Lawer's issue with the non working SRU token was a non-issue, turned out to be a copy-paste problem.
The description of the steps required to get a SRU Access Token was a little too vague:
When requesting the token tell them you want to have access to the title data catalogue ("Titeldaten-Katalog") in the format MARC21-XML. I have updated the
README file accordingly.
Marc21, Marc21-XML and RDF are different data formats, just like JPG and PNG.
DNB offers several catalogues, the Titeldaten-Katalog contains book information (title, author, publication date,...), the GND ("Gemeinsame Normdatei") contains e.g. biographic information about authors, valid descriptive tags etc.