Okay, I'll answer that myself..!
First of all, the Opus doesn't make use of any visible .dat file.
Apparantly you have to download and (then figure out, how to) upgrade the firmware. It includes a MicroSD card (so you better have one ready - or get one quick. Hopefully it's not friday after 18:00, so you'll have to wait for Monday..!)
Once you've done that - all the books that came preinstalled on your Opus are gone.., and the ones you've bought as well..!
But all is not lost, you're a bit wiser as to how the people at Bookeen thinks. It's not something you have any use for, and allthough the problem have existed since 2008 there's no proper warning to try to get access to librarybooks.., oh and by the way, the book transferred now, but it still won't activate..!
Bravo Bookeen. Great job. No matter how this is twisted or turned, I'm seriously waisting time here (and you owe me a freakin' book..!)