I was looking for an explanation of the error shown above.
Its a multisite installation which is embedded into Joomla 1.0.15
The problem appeared after upgrading from 2.1.x to 2.2.6 - the upgrade was done by overwriting all files with the full installation.
I have already tried to overwrite all files again with the full package, but without success.
Gallery-Version = 2.2.6 Kern 184.108.40.206
PHP-Version = 4.4.8 cgi
Webserver = Apache/1.3 (Unix) mod_ssl/2.8.28 OpenSSL/0.9.8f AuthPG/1.3 FrontPage/220.127.116.1135
Datenbank = mysql 3.23.58-log, lock.system=flock
Werkzeuge = Gd, ArchiveUpload
Betriebssystem = Linux quarnero.ispgateway.de 18.104.22.168-grsec #6 SMP Tue Feb 27 09:49:45 CET 2007 i686
Sprache = de_DE
Browser = Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:22.214.171.124) Gecko/2008092417 Firefox/3.0.3