I am using Coreelec for quite some time now, but I cannot solve this issue with a new device of mine (Odroid N2+). I am using two S912 based boxes as well and used the Coreelec backup from one of this boxes to import all the settings (which worked fine) into the basic Odroid-Corelec-install.
The only thing that does not fully work is the artwork. Only some of the covers are showing (the movie info - like content and so on - is there). By the way: I am using a MySQL setup for the database.
I already deleted the local Thumbnail directory and the Textures13.db but without any luck, most of the covers are still missing.
Any idea how to fix this?