[Gpg4win-users-de] Bad secret key

[Felix E. Klee]

>From a backup, I restored a private key:

    gpg2 --allow-secret-key-import --import --armor <key.txt

When I try to move it to a smart card, then this fails with an error
message (GnuPG 2.1.8, libgcrypt 1.6.4):

    gpg: KEYTOCARD failed: Bad secret key

How the private key was created (GnuPG 2.0.19, libcrypt 1.5.0):

    gpg2 --gen-key \
    --personal-digest-preferences SHA512 \
    --cert-digest-algo SHA512

How it was exported as backup:

    gpg2 --armor --export-secret-keys >key.txt

*What is going wrong?*