If you are using vshare 2.6, login to admin area,

Site Settings > Miscellaneous > Create Thumbnail with

What is the current value ? Default it will be ffmpeg-php

The reason for the error is current selected program is not able to find length of video, mostly ffmpeg-php installation error.

Try change it to mplayer or ffmpeg and try again. You can reconvert the video by going to process queue in admin area.