Hi,
Check your php settings for garbage collecting (ie. removing) the sessions.
thanks for the pointer. Actually, the PHP garbage collector script was the culprit, not honoring .htaccess-related changes. So, I did a small workaround
mkdir /etc/php5/mediawiki echo 'session.gc_maxlifetime = 7200' > /etc/php5/mediawiki/php.ini
and it seems to work. Maybe there's a better solution, but at least the above is a workaround. :-)
Thanks! Florian