This is likely happening because your software is out of date, but a quick manual update should resolve the issue. The update feature built into the software will not work for this update, so you will need to download the latest build of the software here:
Make sure to scroll for the version of the software that you own a license for: v6 or v7. Don't worry, your work won't be affected. Once you reinstall Fundy Designer, you should be able to log in with your email and password without issue. If you cannot remember your password, you can reset it in the software!
If you were working in Trial Mode prior to purchasing the software, you will need to go to Help > Log out from the top menu in your application and log back in. The server will reconnect and recognize your new licenses!