For me none of the posted "solutions" worked.
I tried the Option+Command+click on either the Aperture program app or the Aperture library in Finder...did not work.
I tried deleting ~/Library/Preferences... did not work. This was probably the "standard" Apple tech solution.
The solution for me was to Option+click the Aperture program to launch it and Create a NEW empty library.
Then simply IMPORT the corrupted library, in order to recover my photos.
I saw a status message saying "Upgrading Library: scanning library" and lots of CPU activity for a few hours on a 36 GB library.
Several error messages came up about re-publishing previously published galleries but I ignored them (since they were already published)
Not sure what gets lost but at least I got all my photos back.
Anthony Maw, Vancouver, Canada, www.anthonymaw.com