Gnus v5.13
GNU Emacs 23.0.60.1 (i486-pc-linux-gnu, GTK+ Version 2.12.11)
 of 2008-11-30 on turtle, modified by Debian
200 news.motzarella.org InterNetNews NNRP server INN 2.4.5 ready (posting ok).

I'm experiencing painful, but not always reproducible problems with
sending or previewing signed mails.  Often I get an error "Sign failed"
and a backtrace like this:

,----
| Debugger entered--Lisp error: (error "Sign failed")
|   signal(error ("Sign failed"))
|   byte-code("<snipped>" [mml2015-epg-secret-key-id-list error password-cache-remove signal] 4)
|   mml2015-epg-sign((part (sign . "pgpmime") (tag-location . 186) (contents . "Nur ein Test.\n")))
|   mml2015-sign((part (sign . "pgpmime") (tag-location . 186) (contents . "Nur ein Test.\n")))
|   mml-pgpmime-sign-buffer((part (sign . "pgpmime") (tag-location . 186) (contents . "Nur ein Test.\n")))
|   mml-generate-mime-1((part (sign . "pgpmime") (tag-location . 186) (contents . "Nur ein Test.\n")))
|   mml-generate-mime()
|   message-encode-message-body()
|   mml-to-mime()
|   mml-preview(nil)
|   call-interactively(mml-preview nil nil)
`----

If this happens during sending, the message actually is signed and sent,
but no copy lands in my archive folder. :-(

I'm using gpg-agent with pinentry-gtk2 for the passphrase.