Hi Moritz,
I export a private ssh key (ED25519) created for one door and try to import it for another door (which had an RSA-2048 key before, not sure if that is relevant though). During import I get a "Not a valid key!" message (if I use export/import through filesystem) or a "Import Failed." message (if I use export/import through clipboard). Both import and export happen on the same phone and trigger app.
Works flawless if I export and import an RSA key.