Thank you very much, that works!
For anyone who also comes across this, here is what I did:
- Settings (mnu) --> Viewer Plugins --> Movie --> Configure...
- Check: Skip over inital black frames (I just left the "Black threshhold" at 95... I am not really sure what it does anyway)
- Then I refreshed the thumbnails - For this I created a hotkey (Ctrl+~) which executes this command: REFRESHTHUMBS - I talk about this here [Folder Thumbnails - Prefer Images Option?)