Can’t get Thumbnails to work. Tried everything

Hi there,

I successfully run Kodi as installed OSMC image on my Raspberry Pi 2.
I use the Kodi Skin (not the new OSMC one). Howeer I tried both but had no effect on my problem.
I added my videos via smb from my Synology NAS.
The videos are shown in the library and play without problems, but I can’t get
Kodi to generate Thumbnails out of the videos. I marked all the options for generating
tumbnails in the menu, refreshed library several time, but all I get is that camere-symbol
on grey ground on each video. No thumbnails are extracted.

I did long research in the net, figured out that alot of people have those problems,
but I do not found any solution.

Please help. There must be a way to