An error in the translation of the error message above:
"To try again, quit Finder and choose Log Out from the Apple menu."
should be:
"To try again, quit SuperDuper! and choose Log Out from the Apple menu."
This looks very similar to the problem in this thread:
http://www.shirt-pocket.com/forums/s...ead.php?t=2706 where shut down would not happen because SD! could not be quit.
On my machine that problem started with Leopard and mysteriously resolved itself after a couple of months. Possibly (but I am not certain) the problem had gone after a 10.5.x full Combo-update.