Same issue here. I contacted Luxtrust and I got this meaningless response by e-mail:
Customer Service | LuxTrust S.A. wrote:
Hello,
Our App no longer supports Devices or Operating Systems that do not natively use google services, as is the case with GrapheneOS. I’m afraid you will need to switch to a different phone, tablet, or acquire a LuxTrust Scan device through our website.
I recommend anybody affected to contact them:
https://www.luxtrust.com/en/contact-us
According to Luxtrust's own page they had 700k+ users in 2023. Probably more now.
This is a serious issue for GrapheneOS: Luxtrust is the authentication mechanism used by all major banks based in Luxembourg. It is also used to access the on-line services of the public administration.
In practice, this makes GraphneOS hardly usable as a single telephone for hundreds of thousands of people with a relationship with Luxembourg.