OK – for sure the problem is related to Facebook being enabled on a photo library. I share my photo library between iPhoto and Aperture. I suspect using a photo library with Aperture only and Facebook enabled would also cause the problem.
This problem exhibits itself when Aperture crashes after converting a photo database to the latest edition. Trashing the com.apple.Aperture.plist file does not solve the problem by itself. A never ending "opening photo libary" message occurs after the com.apple.Aperture.plist file is removed.
To resolve the issue the photo library must not have Facebook account(s) enabled. Here's the fix...
1. Disable Facebook in System Preferences – there are 2 locations in Sys Pref to remove Facebook: Mail, Contacts & Calendars and Security & Privacy.
2. Open your photo library in iPhoto. Go to Preferences : Accounts. Delete Facebook account(s). This will not delete your pictures on Facebook, it will only remove the connection. When Apple has updated Aperture to fix this problem you can re-establish the link.
3. Quit iPhoto.
4. In the Finder hold down the Option key, select Go from the Finder menu at the top of the window and select Library. Go to Preferences in Library and move com.apple.Aperture.plist to the trash. Alternatively you could open the plist in Text Edit and edit the plugin settings. A prior post in this chain provided those details.
5. Start Aperture and open up the the shared photo library. All should be good.