Decrypt sent mails fails sometimes

[Memnon Anon]

Hi!

I have discovered a strange problem.

Opening some encrypted emails from my Send-Mail fails.
Answering 'Decrypt pgp-part' with yes, I get a buffer like this:

,----
| From: Memnon Anon <gegendosenfleisch@gmail.com>
| Subject: This was the subject
| To: Friend o Mine  <address@address.com>
| Date: Sat, 20 Jun 2009 11:11:11 +0200
| User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/23.0.91 (gnu/linux)
| 
| [1. application/pgp-encrypted]...
`----

Mantra is not prompted for ...
Some Mails just work fine: Same gpg key, same addressee. 
Using gnus in emacs22 or in emacs23.0.91.1 makes do difference ...

I am puzzled ...

Any idea?