After you restore your Library from a vault in Aperture, you may not be able to update the vault again. This can happen if you moved or renamed the vault prior to restoring it—Aperture will look for the vault by its previous name and location.
To resolve this issue, use the Update Vault Path command to point Aperture to the new vault name/location. To do this, just click the gear icon in the lower-left area of the main window, choose Update Vault Path from the pop-up menu, and complete the navigation dialog.
This issue is resolved in Aperture 1.5.