In the meantime (2018) I've updated the plugin a few times and fixed a few bugs some helpful guys reported to me.
It's now also available in a Python 3 variant:
https://github.com/citronalco/calibre-dnb/releases
Maybe I should note that the DNB is the German central archival library. Every publisher is required to hand them a book over for archival. So it's the largest metadata source for German literature.