Do you have any caching plugins installed that may be serving older versions of your site information? If so, be sure to clear them as well as your browser cache before retesting. To be certain, you may want to manually remove cache files via ftp.
Did you create a new Twitter app using your new Twitter account? If you de-authorized your first Twitter account, all its functionality will have been disabled as well.
If those suggestions don't solve it, there may be a conflict with another of your installed plugins. Have you installed anything new recently? Deactivate all plugins except WordTwit then test if you're able to add your new account.