I had that problem once.. I resolved it by navigating to where it downloads the videos, and deleting that specific video file folder. MyTube apparantly keeps all downloaded videos in a folder in a folder format.

So it's like

Mytube > Videos > then..

Video 1
Video 2
Video 3

But all with names like 1sas445 (general format you'd get from downloading a youtube video). Just navigate thru the folders till you find one without a video file. That's the entry you wanna delete (delete the entire folder for it).