Fatal error: Call to a member function getUserId() on a non-object in multisite install

Hi,

I'm in the process of migrating a multisite install from one server to another, and changing the backend database from PostgreSQL to mySQL, as described in http://toddgee.com/node/14.
Everything went fine for the first site (except of some missing modules, which I could fix by installing / activating them), but now I'm stuck while trying to migrate the second site.
I get a Call to a member function getUserId() on a non-object, so, I searched the forum and found the post http://gallery.menalto.com/node/68766, which also deals with such a problem (unfortunately not really my one).
But, as suggested in this thread, I've added some debugging code, and here's what I get now when connecting to the gallery:

=========================/snip/=================================================
Error
realpath(/var/www/galleries-dark-hackers/modules/core/classes/../../../) realpath(/var/www/gallery-darc-o54-de/) Error (GALLERY_ERROR)

* in modules/core/classes/Gallery.class at line 246 (GalleryCoreApi::error)
* in modules/core/classes/helpers/GalleryUserGroupHelper_simple.class at line 37 (Gallery::getActiveUserId)
* in modules/core/classes/GalleryCoreApi.class at line 1776 (GalleryUserGroupHelper_simple::isUserInSiteAdminGroup)
* in modules/core/ErrorPage.inc at line 252 (GalleryCoreApi::isUserInSiteAdminGroup)
* in main.php at line 691 (ErrorPageView::errorHandler)
* in main.php at line 83
* in main.php at line 3

Fatal error: Call to a member function getUserId() on a non-object in /var/www/galleries-dark-hackers/modules/core/classes/Gallery.class on line 252
=======================/snap/=======================================================

I hope someone can tell me what's wrong...

Have a nice hackin',

Harald

Gallery-Version = 2.3.1 Kern 1.3.0.1
API = Core 7.54, Modul 3.9, Motiv 2.6, Eingebettet 1.5
PHP-Version = 5.2.6-1+lenny4 apache2handler
Webserver = Apache/2.2.9 (Debian) PHP/5.2.6-1+lenny4 with Suhosin-Patch
Datenbank = mysqli 5.0.51a-24+lenny2+spu1, lock.system=flock
Werkzeuge = Getid3, LinkItemToolkit, Thumbnail, Exif, ArchiveUpload, NetPBM, Gd, SquareThumb
Beschleunigung = none, none
Betriebssystem = Linux h1685471.stratoserver.net 2.6.18-028stab064.7 #1 SMP Wed Aug 26 13:11:07 MSD 2009 i686
Standard-Motiv = matrix
gettext = Aktiviert
Sprache = de_DE