Subkeys export to Security Token fails: Secret key available.

[Thomas Schneider]

Hi,
I had to reset my blocked Yubikey.
Then I started with setting up the key again; all worked fine including
"key attributes".

After this I tried to export the PGP keys to the token, however this
fails with error message:
gpg: KEYTOCARD failed: Unusable secret key

I don't understand how to fix this issue, and I don't understand what's
causing this issue.
When I execute "gpg --expert --edit-key 0x<key id> I can see this:
Secret key available.

pub rsa4096/Secret subkey is available.

pub rsa4096/<key id>
created: 2020-01-06 expires: 2021-01-05 Nutzung: C
Trust: unbekannt Validity: unbekannt
ssb rsa4096/<key id>
created: 2020-01-06 expires: 2021-01-05 Nutzung: A
Card number:0006 <card id>
ssb rsa4096/<key id>
created: 2020-01-06 expires: 2021-01-05 Nutzung: S
Card number:0006 <card id>
ssb rsa4096/<key id>
created: 2020-01-06 expires: 2021-01-05 Nutzung: E
Card number:0006 <card id>

All subkeys are marked as Stub which is correct because the keys have
been exported before.
However now the keys don't exist anymore on the keycard.

Can you please advise how to fix this issue?

THX