Screensaver starts then immediately stops

Any ideas what could be causing my screen saver to malfunction? It initiates at the correct time interval, as the screen goes dark briefly, but then it immediately stops and returns to the normal display. This is on 10.6.8

Here's the Console log:
28/08/2011 14:14:40 ScreenSaverEngine[10223] _mthid_copyDeviceInfo(216172782187184128) failed