I don't get an exception anymore, but the data isn't correctly 
displayed either.

This is from a monthyear field, the data in the database is 
'a:2:{s:5:"month";s:1:"4";s:4:"year";s:4:"1997";}', but the data in 
the UI is displayed as "0, 0".

Changes have been made in Git (master):

commit cd0653db6285c6f407f1f85522fef8721bc3fcb8
Author: Ralf Lang <lang@b1-systems.de>
Date:   Wed Dec 12 09:18:00 2012 +0200

     Cope with serialized date formats in txt_datavalue for backward 
compatibility #11865.

  sesha/lib/Entity/Value.php |    9 ++++++++-
  1 files changed, 8 insertions(+), 1 deletions(-)

http://git.horde.org/horde-git/-/commit/cd0653db6285c6f407f1f85522fef8721bc3fcb8

I think this results from your previous attempt before the last 
commits but I have to fix it anyway to maintain compat with older data.
Can you dump a test case from you sesha_% tables or is this literally 
what's in the DB?

Ein schwerer Fehler ist aufgetreten:
Zeitformat 
(a:3:{s:5:"month";s:1:"1";s:3:"day";s:1:"1";s:4:"year";s:4:"2000";}) 
nicht erkannt

1. Sesha_Entity_Value->getDataValue() 
/home/jan/horde-develop/sesha/stock.php:128
2. Horde_Date->__construct() 
/home/jan/horde-develop/sesha/lib/Entity/Value.php:27

Details
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Horde_Date_Exception Object
(
     [details] =>
     [logged] => 1
     [_logLevel:protected] => 0
     [message:protected] => Zeitformat 
(a:3:{s:5:"month";s:1:"1";s:3:"day";s:1:"1";s:4:"year";s:4:"2000";}) 
nicht erkannt
     [string:Exception:private] =>
     [code:protected] => 0
     [file:protected] => 
/home/jan/horde-develop/framework/Date/lib/Horde/Date.php
     [line:protected] => 251
     [trace:Exception:private] => Array
         (
             [0] => Array
                 (
                     [file] => 
/home/jan/horde-develop/sesha/lib/Entity/Value.php
                     [line] => 27
                     [function] => __construct
                     [class] => Horde_Date
                     [type] => ->
                     [args] => Array
                         (
                             [0] => 
a:3:{s:5:"month";s:1:"1";s:3:"day";s:1:"1";s:4:"year";s:4:"2000";}
                         )

                 )

             [1] => Array
                 (
                     [file] => /home/jan/horde-develop/sesha/stock.php
                     [line] => 128
                     [function] => getDataValue
                     [class] => Sesha_Entity_Value
                     [type] => ->
                     [args] => Array
                         (
                         )

                 )

         )

     [previous:Exception:private] =>