[gkkiscus]

Oh thanks. Had the feeling that I read it somewhere but didn't find it anymore. Since the last update of the first post was in 2016, I was not sure if the tl;dr sying "make sure jb is up to date" really implies that the provided jb is up to date... But 5.6.1.1, which seems to be the newest update for my 5th gen kindle is rather old as well, probably answers the question.

My main problem right now is, that all of my old collections do not show up anymore since I did the downgrade. I checked with the collection manager from ixtab, and the books show that they are assigned to a collection. However, the collections do not show up. Think I will have to do a factory reset to get at least rid of these ghost-collections

edit:
After updating my kindle back to 6.5.1.1 I could see the collections in the "Cloud" menu of my kindle and was able to simply copy them back to the device.