Hi Alexander.
I'm glad my temporary workaround was helpful for you too. I haven't tried editing any of the template files outside Moodle but it sounds like you might have found a further slight problem here.
The original problem is still something of a mystery for me. I haven't had time to investigate further, but I suspect some slight difference between the zip archive specification Moodle uses and that used by common archive managers. I'd still be interested to here from anyone more familiar with the internal working of the Database module.
Regards,
Ian.