It's possible that you encounter some kind of cache errors in your Contacts. To fix it, try going to Menu > Settings > Applications > All tab and search for the Contacts application. Tap on it and select Clear Cache. After you did this, try again to edit a contact. If you still can't, try downloading an application that can help you solve this problem. I suggest you use Contact Editor. This application is specially designed for this kind of job and it's free. It should fix your problem.