LW usually uses your default media player of your OS to preview videos. Unfortunately, some can't view files if they haven't been rendered, which is the case with video files in uploading progress.
I reckon you download and install the free VLC player (from here):
http://www.videolan.org/vlc/
With this player you will be able to previes files that are uploading choosing them directly from the "Incomplete" folder and not by using the LW preview function.
