Cannot decrypt file encrypted with enQsig

[Felix E. Klee]

To receive a document in encrypted form, I provided my public key to the
sender. See attachment. The key contains a sub key for encryption:

    sec  rsa4096/BEF6EFD38FE8DCA0
         created: 2016-12-17  expires: 2018-12-17  usage: SC
         card-no: 0005 00004980
         trust: ultimate      validity: ultimate
    ssb  rsa4096/04FDF78D1679DD94
         created: 2016-12-17  expires: 2018-12-17  usage: E
         card-no: 0005 00004980
    [ultimate] (1). Felix E. Klee <felix.klee at inka.de>

The sender then prepared the encrypted file using a software called
enQsig: “wir verwenden eine zentrale Gateway Verschlüsselungslösung
(EnQsig).” (German)

After I received `encrypted.asc` from the sender, I tried to decrypt it,
to no avail:

    C:\Users\Felix\Desktop>gpg -v -d encrypted.asc
    gpg: armor header: Version: enQsig
    gpg: public key is BEF6EFD38FE8DCA0
    gpg: no running gpg-agent - starting 'C:\Program Files (x86)\Gpg4win
    \..\GnuPG\bin\gpg-agent.exe'
    gpg: waiting for the agent to come up ... (5s)
    gpg: waiting for the agent to come up ... (4s)
    gpg: connection to agent established
    gpg: pinentry launched (9620 qt 1.1.1-beta5 - - -)
    gpg: public key is 04FDF78D1679DD94
    gpg: using subkey 04FDF78D1679DD94 instead of primary key BEF6EFD38F
    E8DCA0
    gpg: pinentry launched (4608 qt 1.1.1-beta5 - - -)
    gpg: public key is 92663E7CA68E4EC6
    gpg: public key is 9D8C454A43A6D2DE
    gpg: encrypted with RSA key, ID 9D8C454A43A6D2DE
    gpg: encrypted with RSA key, ID 92663E7CA68E4EC6
    gpg: using subkey 04FDF78D1679DD94 instead of primary key BEF6EFD38F
    E8DCA0
    gpg: encrypted with 4096-bit RSA key, ID 04FDF78D1679DD94, created 2
    016-12-17
          "Felix E. Klee <felix.klee at inka.de>"
    gpg: public key decryption failed: Missing item in object
    gpg: encrypted with 4096-bit RSA key, ID BEF6EFD38FE8DCA0, created 2
    016-12-17
          "Felix E. Klee <felix.klee at inka.de>"
    gpg: public key decryption failed: Invalid ID
    gpg: decryption failed: No secret key

>From what I can tell, the file has been encrypted with four keys. My
encryption key is the sub key 04FDF78D1679DD94. The private key is on a
smart card. As you can see, decryption fails with an error message:
“gpg: public key decryption failed: Missing item in object”

*What does the error message mean? Why does encryption fail?*

I wonder if perhaps enQsig cannot properly deal with encryption sub keys:

*Would it be possible to extract the public encryption sub key?* (to
only provide that to the sender)

I am using Gpg4win 3.1.2 on Windows 7x64. If more information is needed,
then I am happy to provide it!
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