Hi all! Thanks for your help! I tried deleting completely the /Fonts folder as Harry suggested, and it worked! The library screen has come back, and I could select and open books again.
I'll email a note to Bookeen, to let them know about the bug.
Thanks again, I'm glad to be able to use this very, very cool reader again.
(But I sure hope they fix the fonts bug in the next 2.0 firmware...)