Hello Andre Völkner,
I see it's quite a while you posted this, did you find a solution in the meantime? I ran into a similar problem.
I have an App (AGOL hosted WAB, not Dev, using the built-in print service) containing secured AGS 10.2.2 Feature services. I created items for them in AGOL with username/pw stored and have also referrer URLs to the webmap and app folders of our AGOL subscription.
All works smooth, except the printing.
I tried to include the AGOL Print Service URL to the referrer URLs, but this doesn't help.
Thanks,
Annina
arcgis online webapp builder
printing widget