I looked for the reason why my videos did not display the thumbnails correctly and did find the reason:

For some reason the VIDEO field is nicely working together with the FileFieldPaths module but the THUMBNAIL field is'nt!

If I set a path where to upload the video and thumbnail to he does upload both correctly and the Dashplayer loads the video from the correct path but in case of the thumbnail he always tries to load from the sites/default/files folder ignoring the specified FFPaths folder!

I'm not expereinced enough to look into and fix it myself.
Can you (someone) please fix this?

Thank you!