KarlKlammer
Just as update from my side:
With App version 5.18.2, they also removed support for the last working version 5.16.1. That means the "downgrade trick" from my post above doesn't work anymore.
Feedback from Developers via Playstore is still: disable secure app spawning. Not very confident, that we will see a solution soon from Postfinance.
Logging in with the TAN generator (yellow "calculator") still works. If I urgently need the app, I temporarily disable secure app spawning, start the app, then enable it again immediately afterwards (app still running). Later I reboot my phone to be on the safe side.
I have other Swiss apps as well that stopped running recently (AGOV), maybe same framework.
My hope is on this comment from GrapheneOS developers: "The app must be using checks for the initial call stack as a poor way of trying to detect hooking by a modified OS. This is trivial to bypass for anything actually doing that. We plan to eventually modify secure spawning to avoid this kind of incorrect security check triggering."
https://discuss.grapheneos.org/d/23378-app-thinks-phone-is-rooted/13