iTunes will not authorize my iPod

My computer will not longer allow me to download some of my songs onto my iPod nano. When I try to sync my iPod a warning pops up that says that some of my purchases are will not be downloaded because my iPod is not authorized on this computer. Then when i try to authorize the computer an unknown error (-42408) occurs.

Just a shot in the dark... maybe you should deauthorize and reauthorize all your computers.