Problem with decrypting a mail with an openpgp card

[Michael Bienia]

Hello,

I've once again a problem with decrypting a mail with my openpgp card.
The used subkey is my encryption key so it should work theoretically.
The out of gnupg (with gnupg-agent) is:
,----
| gpg: armor header: Version: GnuPG v1.4.1 (GNU/Linux)
| gpg: public key is AF58F2B4
| gpg: using subkey AF58F2B4 instead of primary key 968BD587
| gpg: sending command `SCD PKDECRYPT' to agent failed: ec=6.131
| gpg: using subkey AF58F2B4 instead of primary key 968BD587
| gpg: encrypted with 1024-bit RSA key, ID AF58F2B4, created 2006-03-13
|       "Michael Bienia <michael at vorlon.ping.de>"
| gpg: public key decryption failed: general error
| gpg: decryption failed: secret key not available
`----

I can succesfully decrypt an other mail (from someone else) which is
also encrypted to the same subkey (he also used gnupg 1.4.1).

gnupg and gnupg-agent are current versions from the svn (revision 4151).

Has someone an explanation?

TIA,
Michael