Fanarts disappear

I am running OSMC with Kodi 16 on a Rasperry Pi 3. Lately I had a power outage, and after booting the Pi up again, Kodi did not show any fanarts anymore. Everything else was okay, covers were showing, but just black background instead of the fanarts, no matter which skin I used.
So I deleted the whole library, all thumbnails and let Kodi rebuilt the library. Fanarts were there, but disappeared again from one moment to another. When I select to choose the fanart for a movie, it does display the current fanart, so it is there, just not displayed. When I reboot, the fanarts do show again, but sooner or later disappear again and won’t reappear before another reboot.
I have no idea if any other file could have been corrupted or what the issue is. Does anyone have any clue that could help to permanently fix this?