Changes have been made in Git (master):

commit 8a05f074575043ec1cbbf8e684e293c2a8a868ab
Author: Jan Schneider <jan@horde.org>
Date:   Mon Dec 29 14:55:14 2014 +0100

     [jan] Fix Horde_Mime::quotedPrintableEncode() (Bug #13769).

  framework/Mime/lib/Horde/Mime.php |    2 +-
  framework/Mime/package.xml        |    4 ++--
  2 files changed, 3 insertions(+), 3 deletions(-)

http://github.com/horde/horde/commit/8a05f074575043ec1cbbf8e684e293c2a8a868ab

"öäüß" in the subject work (Subject: =?utf-8?b?w7bDpMO8w58=?=), but 
this is another Horde Mime class (Horde_Mime::encode).

I'm not sure the problem is in Ingo or Horde Mime, maybe a Horde 
developer can have a eye on it.

$reason is empty after line 203, I guess Horde_Mime 2.5.0 is the problem.
I see in the Horde_Mime 2.5.0 change log "[mms] Deprecated 
Horde_Mime::quotedPrintableEncode() and move to 
Horde_Mime_QuotedPrintable."


../ingo/lib/Script/Procmail/Recipe.php

199 if (Horde_Mime::is8bit($reason)) {
200        $this->_action[] = '       -i"Subject: ' . 
Horde_Mime::encode($params['action-value']['subject'] . ' (Re: 
$SUBJECT)') . '" \\';
201        $this->_action[] = '       -i"Content-Transfer-Encoding: 
quoted-printable" \\';
202        $this->_action[] = '       -i"Content-Type: text/plain; 
charset=UTF-8" ; \\';
            Horde::debug($reason);
203        $reason = Horde_Mime::quotedPrintableEncode($reason, "\n");
            Horde::debug($reason);
204 } else {
205        $this->_action[] = '       -i"Subject: ' . 
Horde_Mime::encode($params['action-value']['subject'] . ' (Re: 
$SUBJECT)') . '" ; \\';


2014-12-24T17:46:10+00:00 DEBUG: Variable information:
string(13) "Test äüöß"

Backtrace:
1. Ingo_Basic_Base->__construct() /var/www/https/horde/ingo/basic.php:30
2. Ingo_Basic_Vacation->_init() 
/var/www/https/horde/ingo/lib/Basic/Base.php:48
3. Ingo_Script_Util::update() 
/var/www/https/horde/ingo/lib/Basic/Vacation.php:82
4. Ingo_Script_Base->generate() 
/var/www/https/horde/ingo/lib/Script/Util.php:81
5. Ingo_Script_Procmail->_generate() 
/var/www/https/horde/ingo/lib/Script/Base.php:249
6. Ingo_Script_Procmail->generateVacation() 
/var/www/https/horde/ingo/lib/Script/Procmail.php:166
7. Ingo_Script_Procmail_Recipe->__construct() 
/var/www/https/horde/ingo/lib/Script/Procmail.php:296
8. Horde::debug() /var/www/https/horde/ingo/lib/Script/Procmail/Recipe.php:203

2014-12-24T17:46:10+00:00 DEBUG: Variable information:
string(0) ""

Backtrace:
1. Ingo_Basic_Base->__construct() /var/www/https/horde/ingo/basic.php:30
2. Ingo_Basic_Vacation->_init() 
/var/www/https/horde/ingo/lib/Basic/Base.php:48
3. Ingo_Script_Util::update() 
/var/www/https/horde/ingo/lib/Basic/Vacation.php:82
4. Ingo_Script_Base->generate() 
/var/www/https/horde/ingo/lib/Script/Util.php:81
5. Ingo_Script_Procmail->_generate() 
/var/www/https/horde/ingo/lib/Script/Base.php:249
6. Ingo_Script_Procmail->generateVacation() 
/var/www/https/horde/ingo/lib/Script/Procmail.php:166
7. Ingo_Script_Procmail_Recipe->__construct() 
/var/www/https/horde/ingo/lib/Script/Procmail.php:296
8. Horde::debug() /var/www/https/horde/ingo/lib/Script/Procmail/Recipe.php:205

As soon there is a german character in the reason field like ä,ö,ü,ß 
Ingo write
only echo -e "" in the .procmailrc

[...]
-i"Subject: Abwesenheitsnachricht / Out of office (Re: $SUBJECT)" \
-i"Content-Transfer-Encoding: quoted-printable" \
-i"Content-Type: text/plain; charset=UTF-8" ; \
echo -e "" \
[...]

That worked previously without a problem, downgrade ingo doesn't fix 
it. There must be a change somewhere else in the horde code?

Unfortunately I don't know when this broke.