Definitely a corrupt preferences file then. Since it moved forward, and your test account works, I can only assume you reloaded the OS without wiping your drive or you restored, bringing the problem with you on your reload. If that's the case, then do the following:
1. Quit the Mail App.
2. Go to (user) > Library > Preferences and delete this file: com.apple.mail.plist
3. Empty the trash, relaunch the Mail App
This will remove your accounts, so you will have to re-add them. Mail should open, preferences should now work as well.