Importing an off-card backup of the encryption key of a Nitrokey fails with "no user ID"

[sourcelime at mailbox.org]

Hi,

I generated keys on a Nitrokey and have chosen the option to make an 
off-card backup of the encryption key:

gpg: NOTE: backup of card key saved to 
`/home/archi/.gnupg/sk_26D728A8F09033F1.gpg'

as described in: 
https://gnupg.org/howtos/card-howto/en/smartcard-howto-single.html#id2506175

Now I tried to use this backup key. So I imported the public keys, and 
when I try to import the secret key backup file, I get an error "no user 
ID" and the key isn't imported to the secret ring:

gpg2 --import sk_26D728A8F09033F1.gpg
gpg: key 26D728A8F09033F1: no user ID
gpg: Total number processed: 1
gpg:       secret keys read: 1

I only found a hint so far that the key can be uploaded to another card 
with the bkuptocard command 
(https://lists.gnupg.org/pipermail/gnupg-users/2017-June/058438.html), 
but I had hoped that it is possible to use the backup key without a 
card. Any hints here, is this possible?

Greetings,

Ralf