App is ok, but support is lacking a ticketing system.
Finally after three and a half months and several updates later, fingerprint logins function again. The latest update did the trick. It proves that I was right about were the problem was located: in the app. It was difficult to make N26 clear their app was at fault, even though I told them 5 (6 now) other banking apps on my phone had no problems at all with finger print logins, not at first installation, nor at upgrades.