Changes have been made in Git (master):

commit ba55f5e792e7395dcbeadc50aa97c8414c5d8f75
Author: Jan Schneider <jan@horde.org>
Date:   Mon Nov 12 12:40:21 2012 +0100

     We need to set the default owner when importing from iCalendar 
too (Bug #11681).

  nag/docs/CHANGES |    1 +
  nag/lib/Task.php |    5 ++++-
  nag/package.xml  |    2 ++
  3 files changed, 7 insertions(+), 1 deletions(-)

http://git.horde.org/horde-git/-/commit/ba55f5e792e7395dcbeadc50aa97c8414c5d8f75

New tasks should be uploaded to horde/nag upon synchronization. They 
are not. This is the culprit:

2012-11-09T16:40:18+01:00 DEBUG: HORDE [nag] SQL QUERY FAILED: Column 
'task_creator' cannot be null
         INSERT INTO nag_tasks (task_owner, task_creator, task_assignee,
           task_id, task_name, task_uid, task_desc, task_start, task_due,
           task_priority, task_estimate, task_completed, task_alarm,
           task_alarm_methods, task_private, task_parent, task_recurtype,
           task_recurinterval, task_recurenddate, task_recurcount,
           task_recurdays, task_exceptions, task_completions) VALUES
           ('dBETGU6cUytPafXKz0fFUPA', NULL, NULL, 'b45xI06cUytQnSPiNvv35BA',
           'Neue Aufgabe',
           'xxxxx',
           NULL, 0, 0, 0, '0.00', 0, 0, 'N;', 0, '', 0, NULL, NULL, NULL, NULL,
           NULL, NULL) [pid 32321 on line 806 of 
"/usr/share/php/Horde/Db/Adapter/Base.php"]


Could be either a nag problem (task creator needs to allow for NULL 
values) or a sync problem (input from clients has to be parsed for 
NULL values).

This problem appeared after updating from horde 4.x to horde 5.x.

I used Thunderbird / Lightning / Funambol for this.