private subkey not found

[zep]

Hello,

I have some trouble using an imported subkey:

gpg --list-secret-keys

sec   rsa4096/0xABCDEFGH 2015-04-07 [SCA] [expires: 2020-04-05]
      Key fingerprint = ABCD ABCD ABCD ....
uid                   [ultimate] zep <zepmaster at gmx.net>
ssb   rsa4096/0xDEADBEEF 2015-04-07 [E] [expires: 2020-04-05]

So it seems, the private master key (0xABCDEFGH) and the private subkey
(0xDEADBEEF) are both available.

But the decryption of a message encrypted with the subkey fails:

gpg -e -r zepmaster at gmx.net test.txt
gpg -d test.txt.gpg
gpg: encrypted with 4096-bit RSA key, ID 0xDEADBEEF , created 2015-04-07
      "zep <zepmater at gmx.net>"
gpg: public key decryption failed: Invalid name
gpg: decryption failed: No secret key

Also, gpg-connect-agent, then keyinfo --list gives this:

gpg-connect-agent
> keyinfo --list
S KEYINFO "some hex string" D - - - P - - -
ERR 67108891 Not found <GPG Agent>

Has anyone an idea, why decryption with this subkey does not work as
obviously, the private subkey seems to be available.

gpg (GnuPG) 2.1.15
libgcrypt 1.7.3

Thank you very much,

Cheers, zep