SamyGO

Thank you! I'll give it a try tomorrow and report it here.

Okay, I sucessfully enabled characters on serial console.

After that I entered the code posted earlier on this thread:
rm -rf /mtd_contents/V*

After that I got no response of change on console, is that normal?

I attached now my HD to the TV and tried it. The earlier created thumbnails have been removed, but it seems that new thumbnails are not created each time I play a videofile.

Any clue, someone?

EDIT:
just for the record. It seems to be working right now, after playing videofiles just for a few seconds, thumbnails won't be created each time. But playing a file completely, they will.

Thanks to everyone for the support!