vodka Thanks for reporting back, that's useful.
Good to see that Vipps is displaying a slightly more informative error message now, which would have saved some time with previous testing had it displayed this error message previously.
So Vipps has to be installed from Play Store, and possibly you need to be signed in to Play Store when Vipps is checking your device (or more likely, the current profile) for, in their view, compatibility. This rhymes with the recent change to Play Integrity, which requires you to be signed in to Google Play if you want to pass the basic integrity check. My guess is that Vipps has always required that you pass the basic Play Integrity level, and that nothing has changed from their part on that front. But the requirement for it to be installed from Play Store could be a change done by Vipps; that's unclear to me.
The fact that Vipps does not use the Play Integrity API when it displays the error message does not tell us much. It is using the API before that message, and it could just be caching the result and, for some reason, decide to lock you out after some more time has passed.
I am not sure I see this as an "issue" or "bug" that requires a "fix". It seems like a decision from Vipps and/or Google's part. Considering that Vipps already demands that you use Play services, I don't think this it is surprising that Play Store is now also a requirement.
If Vipps decides to use the API to disallow installing the app from the Play Store for non-GMS licensing operating systems, now that is going to be a problem for GrapheneOS users. I am not sure if the recent changes to GrapheneOS would prevent that, as I don't have technical insight into how that is implemented.