We have even tried setting $wgObjectCacheSessionExpiry = 7200; in order to exclude any TimeZone related issues which was our first suspicion. However, changing this from the default 1h session expiry does not change the behavior we're seeing.

I forgot to mention that I understood that the exptime value is inserted into the DB as UTC.

Thus with a default expiry of 1h, it will always show a value earlier than now() at CET without DST.

David