Aperture forgot serial number
I had been using Aperture with no problem last night so I don't know why this has happened.
I used Time Machine to restore the following file from the day before and everything is now OK:
~/Library/Preferences/com.apple.Aperture.plist
Posted in case anyone else has this problem.
Intel iMac, iMac G4, Powerbook G4, iPhone 3G, Apple TV, 3G iPod, 2G Shuffle, Mac OS X (10.6.2), Aperture 2.1.4