Hmmm I went to the page and it seems I need to manually register the app to be able to make api calls. Would I be wrong in assuming the redirect uri needs to be the one I got in the error message popup when trying to sign in through the app?
Edit: seems it doesnt work. I registered an app in openstreetmaps.org with that and nothing changes.
I just downloaded streetcomplete and the login works there. It might be a good idea to file an issue on the github for osmand at this point. Thanks for all your help! :)