As far as I can tell the problem with the old version not starting only happens for users that bought the app with another AppleID than they are running currently on the iPad. The problem can be fixed by switching to the AppleID you used to buy the app and then either waiting for an app update to appear or uninstalling and reinstalling the app.
Reinstalling the app will delete all data. So make sure you copy the most recent backup file to your computer before reinstalling. Restore the backup after reinstalling.
To migrate to version 3 you neet a backup file from version 2. See here: https://stagetraxx.com/user-guide/first-steps/#migrating-from-version-2