Fixed for future updates. For those that are affected, it's indeed the 
easiest to simply re-create the prefs.php file from the .dist versions.

Changes have been made in CVS for this ticket:

http://cvs.horde.org/diff.php/groupware/docs/groupware/CHANGES?r1=1.33&r2=1.34&ty=u
http://cvs.horde.org/diff.php/groupware/docs/groupware/UPGRADING?r1=1.8&r2=1.9&ty=u
http://cvs.horde.org/diff.php/groupware/docs/webmail/CHANGES?r1=1.30&r2=1.31&ty=u
http://cvs.horde.org/diff.php/groupware/docs/webmail/UPGRADING?r1=1.10&r2=1.11&ty=u
http://cvs.horde.org/diff.php/groupware/patches/config/horde-prefs.php_1.0_1.1.php?r1=1.2&r2=1.3&ty=u
http://cvs.horde.org/diff.php/groupware/patches/config/kronolith-prefs.php_1.0_1.1.php?r1=1.2&r2=1.3&ty=u
http://cvs.horde.org/diff.php/groupware/patches/config/turba-prefs.php_1.0_1.1.php?r1=1.2&r2=1.3&ty=u
http://cvs.horde.org/diff.php/groupware/patches/webmail/config/imp-prefs.php_1.0_1.1.php?r1=1.1&r2=1.2&ty=u

Still a good idea, works fine!

I get the same failure after the upgrade. I solved the problem by 
replace the file imp/config/prefs.php from the original source 
ftp://ftp.horde.org/pub/horde-webmail/horde-webmail-1.2.tar.gz .

I get two errors:
Horde -> Options -> Webmail -> Search and Addressbook

Search Options

Warning: 
Prefs_UI::require(/var/www/https/horde/config/../imp/templates/prefs/defaultsearchselect.inc) [function.Prefs-UI-require]: failed to open stream: No such file or directory in /var/www/https/horde/lib/Horde/Prefs/UI.php on line 
256

Fatal error: Prefs_UI::require() [function.require]: Failed opening 
required 
'/var/www/https/horde/config/../imp/templates/prefs/defaultsearchselect.inc' 
(include_path='/var/www/https/horde/lib:/var/www/https/horde/lib/../pear') in 
/var/www/https/horde/lib/Horde/Prefs/UI.php on line 256

AND

Adressbücher

Warning: 
Prefs_UI::require(/var/www/https/horde/config/../templates/prefs/.inc) 
[function.Prefs-UI-require]: failed to open stream: No such file or 
directory in /var/www/https/horde/lib/Horde/Prefs/UI.php on line 260

Fatal error: Prefs_UI::require() [function.require]: Failed opening 
required '/var/www/https/horde/config/../templates/prefs/.inc' 
(include_path='/var/www/https/horde/lib:/var/www/https/horde/lib/../pear') in 
/var/www/https/horde/lib/Horde/Prefs/UI.php on line 260

So I change my /horde/imp/config/prefs.php


//$prefGroups['search'] = array(
//    'column' => _("Other Options"),
//    'label' => _("Search Options"),
//    'desc' => _("Control options relating to messages searching."),
//    'members' => array('defaultsearchselect')
//);

$prefGroups['filters'] = array(
     'column' => _("Other Options"),
     'label' => _("Filters"),
     'desc' => _("Create filtering rules to organize your incoming 
mail, sort it into folders, and delete spam."),
     'url' => 'filterprefs.php'
);

//$prefGroups['addressbooks'] = array(
//    'column' => _("Other Options"),
//    'label' => _("Address Books"),
//    'desc' => _("Select address book sources for adding and 
searching for addresses."),
//   'members' => array('auto_expand', 'save_recipients', 
'display_contact', 'sourceselect')
//);

So whats going wronge there?
Won't install new, I need the Users files with all the addressbooks 
and options.

I have the same problem with these options after updating (without 
errors) to horde-webmail-1.2:
Global Options -> Display Options
Options for Address Book  -> Display
Options for Calendar -> Free/Busy Information
Options for Mail  -> Mailbox and Folder Display Options
Options for Mail -> Search Options
Options for Mail  -> Address Books
It seems as if the file /lib/Horde/Prefs/UI.php does not get the 
type... My temporary solution was to edit the prefs.phps and remove 
the concerning members.

Its the same!
unset($prefGroups['addressbooks']['members']['auto_expand'],
       $prefGroups['identities']['members']['mail_hdr'],
       $prefGroups['compose']['members']['num_words'],
       $prefGroups['newmail']['members']['nav_audio'],
       $prefGroups['display']['members']['show_legend'],
       $_prefs['preview_maxlen']['enums'][-1],
//      $_prefs['show_legend'],
//      $_prefs['defaultsearchselect'],
       $_prefs['default_search'],
       $_prefs['auto_expand']);


Warning: 
Prefs_UI::require(/var/www/https/horde/config/../templates/prefs/.inc) 
[function.Prefs-UI-require]: failed to open stream: No such file or 
directory in /var/www/https/horde/lib/Horde/Prefs/UI.php on line 260

Fatal error: Prefs_UI::require() [function.require]: Failed opening 
required '/var/www/https/horde/config/../templates/prefs/.inc' 
(include_path='/var/www/https/horde/lib:/var/www/https/horde/lib/../pear') in 
/var/www/https/horde/lib/Horde/Prefs/UI.php on line 260

Try adding
$prefGroups['addressbooks']['members']['auto_expand'],
to the first unset() call of the configuration changes added for version 1.1.

The update script show some notes, but no errors. Sorry I forgotten to 
save this informations.
I uploade me prefs.php as attachment

Did the upgrade script succeed without any errors? How does the 
generated imp/config/prefs.php file look like?

I make an upgrade to 1.2 final.
IMP -> Options -> Webmail -> Adressbook
I get following error:
Warning: 
Prefs_UI::require(/var/www/https/horde/config/../templates/prefs/.inc) 
[function.Prefs-UI-require]: failed to open stream: No such file or 
directory in /var/www/https/horde/lib/Horde/Prefs/UI.php on line 260

Fatal error: Prefs_UI::require() [function.require]: Failed opening 
required '/var/www/https/horde/config/../templates/prefs/.inc' 
(include_path='/var/www/https/horde/lib:/var/www/https/horde/lib/../pear') in 
/var/www/https/horde/lib/Horde/Prefs/UI.php on line 260