I downloaded a few books via Calibre. I placed them on my Kobo according to the Kobo instructions. This all went well.
Unfortunately, all read books, except for 2, are now marked as unread.

The read books are no longer on the Kobo. I can still see the covers.
Does anyone have an idea why this happened? Can this be fixed? Yes, I can go through all the books one by one, but that can't be the intention, can it?
There are many problem solutions on the Kobo site, but I can't find this one. Not on other sites either.
This site is supposed to be leading, so I'll try the question here first.
Regards
Rob