I've emailed Bookeen, but with the weekend... I have to wait a couple of days before response.
Luckly SD memory works okee and allows to remember where I'm in a book and even bookmarkings works with it. This wasn't working from the start with books in internal memory.
I'm now more and more convinced that I have to reformat the internal disk/memory and reinstall the software

but will wait for Bookeens response before doing such a drastic action.