I'm having the same problem. You can open the screensavers using the space bar and it plays just fine. I'm wondering if it has to do with the System preferences app not having enough memory to generate a preview.
As anyone found a way to create a video playing screen saver in Quartz Composer in Snow Leopard. These screen savers work fine in Leopard but crash System Preferences when you attempt to load them in Snow Leopard. They seem to work just fine within Quartz Composer. Any insight will be appreciated. Thanks.

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