Well, I have tried the newest version of Duokan with same results.
I have sent them questions/feedback to the given email-address.
At the moment it looks like I am going to sell the K4NT again.
Interestingly, when turning pages in the menu or when viewing a book, right before the page turns, the fonts can be seen really black for a few milliseconds. That's the way I want it to look. With Duokan 0.5.13.XXXX text can be constantly viewed like this, real black on white.
I wonder why this important option is disabled in Duokan 2012. It could have better contrast theoretically but it would have to be implemented in the firmware too. No matter what settings I change, text always looks kind of grey to me in Duokan 2012. That's a pitty.