Nitrokey 3A: gpg keytocard fails

Has someone managed to transfer a RSA 4096 key to the nitrokey device? I changed the key attributes on the card with key-attr to rsa4096 but when I try to tranfer the key with keytocard, I get the error message: “gpg: KEYTOCARD failed: Unusable secret key”.

In the meantime, I was able to solve the problem. I deleted the keys from the system and re-imported them from the backup, so there’s no card number mentioned. After that, keytocard was successful.