gpg: [don't know]: 1st length byte missing

[Felix E. Klee]

See the attached file. When I try to decrypt it using `gpg -d`, I get:

    gpg: [don't know]: 1st length byte missing

`gpg --version` (on Windows):

    gpg (GnuPG) 2.2.1
    libgcrypt 1.8.1
    Copyright (C) 2017 Free Software Foundation, Inc.
    License GPLv3+: GNU GPL version 3 or later
<https://gnu.org/licenses/gpl.html>
    This is free software: you are free to change and redistribute it.
    There is NO WARRANTY, to the extent permitted by law.

    Home: C:/Users/Felix/AppData/Roaming/gnupg
    Supported algorithms:
    Pubkey: RSA, ELG, DSA, ECDH, ECDSA, EDDSA
    Cipher: IDEA, 3DES, CAST5, BLOWFISH, AES, AES192, AES256, TWOFISH,
            CAMELLIA128, CAMELLIA192, CAMELLIA256
    Hash: SHA1, RIPEMD160, SHA256, SHA384, SHA512, SHA224
    Compression: Uncompressed, ZIP, ZLIB, BZIP2

IIRC I haven’t updated gpg since I encrypted the file. So I assume that
the same gpg 2.2.1 has been used for encryption.

The private key is on an OpenPGP smartcard by ZeitControl.

*Any idea how to fix the issue?*
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