"The expected version of the product was not found on the system"It's very strange; as the current farm version is "12.0.0.6421", which is the version it should be after installing SP2, and once installed those cumulative updates the version should be "12.0.0.6504".
I checked all updates I have installed on the farm, and finally found that I did not install SP2 for the language packs, after installing SP2 for language packs, I ran the cumulative updates and everything works fine.
So you should ensure to install service packs for language packs as its not going to be updated when installing service packs for MOSS itself.
