Thank you very much for the quick response und resolution!

Two problems here.

First, the MIME structure of the embedded signed part was causing an 
off-by-one error with our MIME parsing code, so it was skipping the 
first part.  This has been fixed.

Second, Claws Mail sends broken embedded signed data - it mixes the 
line endings which will confusing MIME parsing.  Workaround this issue 
by normalizing to CRLF endings at parse time.

Changes have been made in Git for this ticket:

[mms] Workaround broken PGP signed data contained within encrypted 
parts (Bug #10809).
This is significant enough to warrant bumping Horde_Mime dependency to
the latest version.

  2 files changed, 5 insertions(+), 1 deletions(-)
http://git.horde.org/horde-git/-/commit/4bea0d28348935429544066ab2c03ba0d03ac41a

Changes have been made in Git for this ticket:

[mms] Workaround incoming message data to 
Horde_Mime_Part::getRawPartText() that contains varying EOL characters 
(Bug #10809).

  2 files changed, 8 insertions(+), 0 deletions(-)
http://git.horde.org/horde-git/-/commit/1ddfb442e3eeb4bb2c357b07f9ab0873228b19ad

Changes have been made in Git for this ticket:

[mms] Fix detection of MIME boundaries at beginning of string (Bug #10809).

  2 files changed, 10 insertions(+), 2 deletions(-)
http://git.horde.org/horde-git/-/commit/6dc3d20b5c4f96e0ec5a266719753f2032469c20

Here it comes.

Anything else you need?

I have similar (but slightly different since the error message is:
"Die Daten in diesem Nachrichtenteil wurden mit PGP digital signiert.
gpg: verify signatures failed: Unerwartete Daten")
problems with all messages I created via enigmail 1.3.3 with Thunderbird 8.0.

I didn't test other clients, but the message I reported about 
originally and this debug is correlated to is from another client.

Thanks for your help.

Here's what I see.

You have a PGP encrypted message.  It looks like this:

(0) multipart/encrypted
   1 application/pgp-encrypted
   2 application/octet-stream

So far, so good.

IMP correctly decrypts the data.  When we do this, we create a 
"virtual" MIME part - Part ID 3.  We need to do this because the 
encrypted data is itself a MIME part, and we need to send this data 
through our viewing system to be
parsed.

Sure enough, the encrypted part is PGP signed.  So our message now 
(should) look like this:

(0) multipart/encrypted
   1 application/pgp-encrypted
   2 application/octet-stream
   (3) multipart/signed
     (3.1) [Message Data]
     (3.2) application/pgp-signature

BUT the problem with your message is that it appears that (3.1) is 
completely missing.  In other words, multipart/signed requires 2 parts 
but your message only contains one.  So that's why part ID 3.2 can't 
be found - it doesn't exist.  And if it doesn't exist, that means that 
your PGP message is broken (it looks like your message contains 
pgp-signature data but no message data).

I can't be sure until you provide the message source of the decrypted 
message.  Easiest way is to do this on line 283 of 
imp/lib/Mime/Viewer/Pgp.php:

Horde::debug($decrypted_data->message);

And post the output here (redacting any sensitive data).

---

Horde::debug() documentation: http://wiki.horde.org/Doc/Dev/DebugH4

I had not as much time as I would have liked to investigate, but at 
least I know this by now:

The error becomes manifest on line 378 in file 
imp/lib/Mime/Viewer/Pgp.php where getMetadata() fails since the 
variable the should be a Horde_Mime_Part is NULL. This is the case 
since  $this->getConfigParam('imp_contents')->getMIMEPart($sig_id); 
results NULL a few lines before.

I inserted the following line for more information about the object involved:

Horde::logMessage('Variable: ' .
                 $sig_id . ' - ' .
                 var_export( 
$this->getConfigParam('imp_contents')->getTree() , true )
                 , Horde_Log::NOTICE);

This logs as follows:

2011-12-04T23:20:42+00:00 NOTICE: HORDE [imp] Variable: 3.2 -
Horde_Core_Tree_Html::__set_state(array(
    '_images' =>
   array (
     'line' => 1,
     'blank' => '',
     'join' => 2,
     'join_bottom' => 4,
     'join_top' => 3,
     'plus' => 10,
     'plus_bottom' => 11,
     'plus_only' => 12,
     'minus' => 6,
     'minus_bottom' => 7,
     'minus_only' => 8,
     'null_only' => 13,
     'folder' => 14,
     'folderopen' => 15,
     'leaf' => 16,
   ),
    '_node_pos' =>
   array (
   ),
    '_dropline' =>
   array (
   ),
    '_altCount' => 0,
    '_allowed' =>
   array (
     0 => 'class',
     1 => 'icon',
     2 => 'iconalt',
     3 => 'iconopen',
     4 => 'url',
     5 => 'urlclass',
     6 => 'title',
     7 => 'target',
   ),
    '_instance' => 'mime-44',
    '_header' =>
   array (
   ),
    '_nodes' =>
   array (
     0 =>
     array (
       'label' => 'Encrypted-Nachrichtenteil (4 KB) ',
       'expanded' => true,
       'class' => 'partsTreeDiv',
       'icon' => '/horde/themes/silver/graphics/mime/encryption.png',
       'children' =>
       array (
         0 => '1',
         1 => '2',
         2 => '3',
       ),
     ),
     1 =>
     array (
       'label' => 'Pgp-encrypted-Nachrichtenteil (1 KB) <a 
href="/horde/services/download/?module=imp&amp;actionID=download_attach&amp;id=1&amp;uid=44&amp;mailbox=SU5CT1guWjpLZWVw&amp;fn=%2F" class="iconImg downloadAtc" target="view_c5a5646e66de90e04d5d253f906b4fce" 
title="Herunterladen"></a>',
       'expanded' => true,
       'class' => 'partsTreeDiv',
       'icon' => '/horde/themes/silver/graphics/mime/encryption.png',
     ),
     2 =>
     array (
       'label' => '<a 
onclick="void(Horde.popup(\'%7B%22url%22%3A%22%5C%2Fhorde%5C%2Fimp%5C%2Fview.php%22%2C%22menu%22%3A1%2C%22params%22%3A%22autodetect%3D1%26actionID%3Dview_attach%26id%3D2%26uid%3D44%26mailbox%3DSU5CT1guWjpLZWVw%22%7D\'));" title="Octet-stream-Nachrichtenteil anzeigen">Octet-stream-Nachrichtenteil</a> (3 KB) <a href="/horde/services/download/?module=imp&amp;actionID=download_attach&amp;id=2&amp;uid=44&amp;mailbox=SU5CT1guWjpLZWVw&amp;fn=%2F" class="iconImg downloadAtc" target="view_810784d836d921f732433c9718a05812" 
title="Herunterladen"></a>',
       'expanded' => true,
       'class' => 'partsTreeDiv',
       'icon' => '/horde/themes/silver/graphics/mime/binary.png',
     ),
     3 =>
     array (
       'label' => 'Signed-Nachrichtenteil (1 KB) ',
       'expanded' => true,
       'class' => 'partsTreeDiv',
       'icon' => '/horde/themes/silver/graphics/mime/encryption.png',
       'children' =>
       array (
         0 => '3.1',
       ),
     ),
     '3.1' =>
     array (
       'label' => 'signature.asc (1 KB) <a 
href="/horde/services/download/?module=imp&amp;actionID=download_attach&amp;id=3.1&amp;uid=44&amp;mailbox=SU5CT1guWjpLZWVw&amp;fn=%2Fsignature.asc" class="iconImg downloadAtc" target="view_8b128f6be45facb705df7d8b586fa4af" 
title="Herunterladen"></a>',
       'expanded' => true,
       'class' => 'partsTreeDiv',
       'icon' => '/horde/themes/silver/graphics/mime/encryption.png',
     ),
   ),
    '_root_nodes' =>
   array (
     0 => '0',
   ),
    '_colsLeft' => 0,
    '_colsRight' => 0,
    '_options' =>
   array (
     'lines' => true,
     'nosession' => true,
   ),
    '_sortCriteria' => NULL,
    '_static' => false,
))


Do you have any idea why the MIMEpart may not exist?
Here the source of the message for reference:

Return-Path: <bjoern.wiedersheim@uni-ulm.de>
Received: ...
         by poseidon (Cyrus v2.3.14) with LMTPA;
         Wed, 20 Oct 2010 12:30:41 +0200
X-Sieve: CMU Sieve 2.3
Received: from stella ...
        by ...
        (version=TLSv1/SSLv3 cipher=DHE-RSA-AES128-SHA bits=128 verify=NOT)
        for <stephan.kleber@...
Date: Wed, 20 Oct 2010 12:30:35 +0200
From: ...
To: Stephan Kleber <stephan.kleber@...
Subject: ...
Message-ID: <20101020123035.19c23241@stella>
Organization: =?ISO-8859-1?B?VW5pdmVyc2l05HQ=?= Ulm
X-Mailer: Claws Mail 3.7.4 (GTK+ 2.20.1; x86_64-pc-linux-gnu)
Mime-Version: 1.0
Content-Type: multipart/encrypted; 
boundary="Encrypt_/tBF.7GiOGOrrodg9BGSUSXM";
  protocol="application/pgp-encrypted"
X-DCC-dmv.com-Metrics: poseidon 1181; Body=1 Fuz1=1
X-Virus-Scanned: by amavisd-new

--Encrypt_/tBF.7GiOGOrrodg9BGSUSXM
Content-Type: application/pgp-encrypted

Version: 1

--Encrypt_/tBF.7GiOGOrrodg9BGSUSXM
Content-Type: application/octet-stream

-----BEGIN PGP MESSAGE-----
Version: GnuPG v1.4.10 (GNU/Linux)

...
-----END PGP MESSAGE-----

--Encrypt_/tBF.7GiOGOrrodg9BGSUSXM--

This is always the case.  If a developer can't reproduce the issue, 
the understanding is that the original reporter needs to provide, at a 
minimum, detailed instructions to reproduce or, better yet, debugging 
to the point where you can point to a developer exactly why the code 
is breaking.  We are not asking for a full patch.  But if a developer 
can't reproduce, they are never going to look at the ticket again.  It 
is extremely difficult to fix something that isn't broken.

...
That sounds more constructive. It will take until the weekend before I 
will have the time to investigate, but I am willing and able - I think 
- to debug, at least to a certain degree. I certainly will need your 
expertise to solve the issue, though.

I appreciate your work and help, but I have to admit, sometimes I have 
the impression you are a bit aggravated on seamingly minor issues.

I hope we can solve this together.

You need to debug the code and determine where it is failing.  PGP 
encrypted messages are notoriously difficult for developers to debug 
since we can not debug a copy of the message source.  It really relies 
on the original reporter.  If you are unable/unwilling to do this, we 
will have to close the ticket since, as mentioned, a developer can not 
reproduce.

I believe you. Nevertheless is this comment not helpful. Do you 
suggest I phantasize?
How can I help track down the issue?

Can't reproduce.

I noticed that I get an error when trying to decrypt a PGP-encrypted
message.
In imp-dynamic when clicking on
"Sie müssen das Passwort für Ihren privaten PGP-Schlüssel angeben, um
diese Nachricht lesen zu können."
I get a red notification saying
"Fehler während der Kommunikation mit dem Server."

Sometimes - indeterminately - not even any error message is displayed 
or logged, but the AJAX-Loading-Circle is showing forever.

In imp-traditional I get a completely empty page and either way the 
following log entry in php-error-log (no entry in horde log):

[29-Nov-2011 04:23:38] PHP Fatal error:  Call to a member function 
getMetadata() on a non-object in 
/var/www-ssl/horde4/imp/lib/Mime/Viewer/Pgp.php on line 378