I've noticed that when no video thumbnail is available, a blank space appears instead of the default video thumbnail.

Here is the fix:

in ow_plugins/video/bol/clip_service.php, replace line 311 from

return OW::getThemeManager()->getCurrentTheme()->getStaticImagesUrl() . 'video-no-video.png';

to

return OW::getThemeManager()->getCurrentTheme()->getStaticImagesUrl() . 'video-no-video.jpg';

Voilà!