Well, after doing some testing, I think Calibre is the cause (when you use it in the wrong way)....
=> I saw that when you just closed Calibre, without ejecting the device, and then unmounted the device in Linux, it had this behavior. I think it caused some problem in the book.db database file....
Everytime I did it in the "wrong" way, my reader started working "weird" (with that force closing message when closing the reader-app, and when going into the settings of the reader-app => to see e.g. details / more info about the book)
Resetting works (beautifully), and now, when properly ejecting the reader first from Calibre, and then unmounting it in my OS, I don't have any problem at all anymore;
so probably just my fault
(not knowing how to use Calibre
)
I learned something,
but yes, sometimes the reader
should do a fsck (every 50 boots at least or so?) to check the integrity of the filesystem; else it will be giving problems in the future!
(or they would have implemented some other way of assuring the integrity of the filesystem - but I doubt about that
)