@Leir: The only way I can reproduce this is if one of the files in the rotation is broken/in the wrong format. It seems to screw up for good (even if the next file is okay) until the next reboot/framework restart.
So, yeah, how many custom screensavers do you have? And check that they're all proper PNG files. (A jpg renamed as a .png is a good way to trigger this kind of thing, for example
).