My fault - not grey, but some 
I-won't-tell-you-what-I-associate-with-it-yellowish color... see 
screen shot attached. This is right after pressing "save" without the 
patch in an account without preferences folder.

There are no grey bars in the top area of Horde, so I have no idea 
*what* has sent you the confirmation. ;-)

Sorry, missed to document one step:
- changed the preference values and clicked "save"

I just re-tested *without* your patch:
- deleted current settings folder of test user via cyradm
- logged in as test user - got "never logged in before" as an 
indication that no pref store is available
- changed the user's identity settings, clicked "save" and received a 
grey bar in the top area with the text
"Ihre Einstellungen wurden gespeichert." - sounds very positive.
- (checked via cyradm: no new folder created)
- logged off and re-logged in the test user
- user identity settings gone.

Then I logged off test user again and applied your patch to the test server:
- checked via cyradm: no prefs folder
- logged in test user, opened page to change user's identity settings as above
- cyradm shows the prefs folder
- "Ihre Einstellungen wurden gespeichert."
- the settings persist across a logoff/logon cycle.

So it's evident that the patch works for me :) and I can reproduce the 
"positive feedback but nothing stored".

And without that, I got a fatal error, so there was no silent saving, 
then discarding for preferences possible at all.

Okay, I'm not sure why, but this is explicitly disallowed. Prefs work 
fine for me with the committed fix for creating the preference folder 
though.

no, with a non-admin user account. As Horde always tells me after an 
admin login that the preferences subsystem is unavailable, I hadn't 
given that any chance :)

Changes have been made in Git (master):

commit f186746986393b1aaf3077e6ec02fc112dcde809
Author: Jan Schneider <jan@horde.org>
Date:   Wed Nov 28 13:47:19 2012 +0100

     [jan] Fix creating storage folder for Kolab/IMAP backends (Bug #11751).

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

http://git.horde.org/horde-git/-/commit/f186746986393b1aaf3077e6ec02fc112dcde809

Did you test this with an administrator user?

Today I had to drop my (even pre-H4-) IMAP-based user preferences, 
which seem to have caused severe problems with various Horde5 
components. I did so by deleting the IMAP folder 
"Benutzereinstellungen", a direct child of "INBOX" on my Cyrus IMAP 
server. I'm using Kolab integration.

While zapping those old preferences helped fixing a lot of other 
problems, I had more than a hard time to get new preferences stored: 
After logging in with my user account. entering new configuration 
values and receiving positive acknowledgment when selecting to store 
them, they still were all gone once I logged off and logged in again.

Only after I manually re-created the folder "Benutzereinstellungen" 
(from within H5/imp), preferences were store in the typical Kolab 
style of IMAP files.

Why did I have to recreate that folder, rather than the preferences 
subsystem creating it when storing preferences? Why did I receive 
positive acknowledgment when storing the preferences, although no 
preferences were available after logging off/on?

How's the folder to store the preferences in selected? May they have 
been stored in some other user's preferences folder? If so, why 
weren't those preferences re-read after logging in with my original 
folder deleted? (I've seen one case of cross-over preferences stored 
in another user's folder  I had write permissions for. That user saw 
"my" identities, but obviously as an individual copy, as deleting 
those identities in that account didn't influence those in my 
account...)