Just a thought here...
There's a known bug where in System->Screensaver->Preferences, the demos do not show the correct size, so you only see part of the image. This results in a pure white screen for the Eternity plugin, but only in the demo screen. If you let the screensaver actually trigger for real, then it works fine. Just to be sure, does this problem only affect then demo or do you still get a white screen when the screensaver triggers for real?
Note that the screensaver should work with or without compiz/beryl, but they can affect things and I was just trying to work out what was different about your system.