This is the topic that had me delete the calibre files. Back then it was a common problem that Calibre an Kobo Touch didn't play nicely, but, like I said, that should be fixed by now.
https://www.mobileread.com/forums/sho...d.php?t=143654
It does make sense that those files should not matter for how the device sees the books, but rather how Calibre sees them Nevertheless my problem back then
was that the device didn't see the book (even though Calibre and the file explorer said it was there).