@ixtab/diba: If it's still like on the K3, check the logs when the framework throws a 'can't open this book' popup, and it will probably complain about failing to find/open/parse a font.
I have no idea how or why that happens, because a simple framework restart 'fixed' it [without switching fonts/ft engine], but it did happen to me a couple of times a whiiiile back on the K3 when playing a bit too much with the [Aa] menu and custom fonts... (I don't even remember if I had started using a custom ft lib at that time :/).
I haven't seen it since, though, but then I almost never open the Aa menu anymore

.