There is a long-standing bug in Mac OS X 10.6 and later which causes video screensavers to exit immediately. This problem affects all screensavers, not just iScreensaver and it is truly a bug in the OS.
There is a workaround: re-export your video file in another codec. If it is H.264 then re-exporting as H.264 again will usually fix the issue. You can also try other codecs such as PhotoJPEG (large file size, but very fast and low CPU usage).
For more details, see http://iscreensaver.com/releaseNotes