Changes have been made in Git (master):

commit 8a066062668bd32a7a970339433e882dcee1cba5
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Sun Mar 17 15:59:33 2013 -0600

     Catch bad prefs.php configs (Bug #12121)

  framework/Core/lib/Horde/Core/Prefs/Ui.php |    5 +++++
  1 files changed, 5 insertions(+), 0 deletions(-)

http://git.horde.org/horde-git/-/commit/8a066062668bd32a7a970339433e882dcee1cba5

Probably still makes sense to put something in the UI code to throw a 
more coherent error message.

Spot on! There was an incorrect entry in 'config/prefs.local.php' 
(lesson learned, only override things that need to be changed). I had 
looked for problems in 'imp/config/prefs.local.php' (since the 
problems occurred only when changing e-mail preferences), but 
completely missed this one. Case closed.

This would indicate you have a broken prefs entry (prefs entries that 
are displayed on the page MUST have a 'type' key).

When running with logging DEBUG, I see the following being logged at 
the time I enter the page where identities are managed:

2013-03-16T21:15:23+01:00 DEBUG: horde [imp] Load config file 
(prefs.php; app: horde) [pid 11287 on line 480 of 
"/usr/share/php5/PEAR/Horde.php"]
2013-03-16T21:15:23+01:00 WARN: horde [imp] PHP ERROR: filemtime(): 
stat failed for /srv/www/htdocs/horde/lib/../templates/prefs/.html 
[pid 11287 on line 233 of "/usr/share/php5/PEAR/Horde/Template.php"]
2013-03-16T21:15:23+01:00 DEBUG: horde 1. 
Horde_Core_Prefs_Ui->generateUI() 
/srv/www/htdocs/horde/services/prefs.php:32
2. Horde_Template->fetch() /usr/share/php5/PEAR/Horde/Core/Prefs/Ui.php:579
3. filemtime() /usr/share/php5/PEAR/Horde/Template.php:233
4. Horde_ErrorHandler::errorHandler()

2013-03-16T21:15:23+01:00 DEBUG: horde [imp] SQL  (0,0010s)
         SELECT g.group_uid AS group_uid, g.group_name AS group_name FROM
           horde_groups g, horde_groups_members m WHERE m.user_uid = 
'arjen' AND
           g.group_uid = m.group_uid ORDER BY g.group_name [pid 11287 
on line 806 of "/usr/share/php5/PEAR/Horde/Db/Adapter/Base.php"]

Could this have to do something with this?

Can't reproduce on either IMP 6.0.5 or 6.1.0.

A signature can be set in the IMP preferences through

     http://www.example.com/services/prefs.php?app=imp&group=identities

This works fine and the signature is attached for the identity it is 
set for. The problem is, when revisiting the identity, the signature 
field is now empty. It is therefor no longer possible to modify the 
signature, nor remove it once it is set. It will be appended 
nevertheless. The only way to remove it, is by changing it directly in 
the MySQL database.