First I want to thank everybody chiming in here. I really appreciate it.
a small update: On Friday we were able to change the “primary url” of the Hamburg-publication in Google News with the help of Google News support. It looks like this might have been the root cause. Because since Saturday the traffic has a small uptick.
Some background why we didn’t do that from the start: On and they disabled immediately disabled the option to create new publications and this included disabling the possibility to change the primary URL of already existing publications.
I don’t know why impacts canonical URLs and why they didn’t already switch to automatically generated publications (like they announced they will this year).