Screen saver password protect not working

I have a 2003 Server network of about 40 PCs. Yesterday, for no apparent reason, the password protect on the screen saver stopped working, on both the primary and secondary domain controllers, and at least some (haven't checked them all) of the PC on the network. The screen savers come up as usual after the delay period. The "on resume, password protect" box is checked, but when it returns from the screen saver, there's no password required. I edited the registry on the primary DC to set the grace period to zero, and nothing changed. I also set group policy to force the box to be checked, that made no difference. Rebooted both DC's, no change. Any ideas? Thanks.