Apple changed encryption keys in the iOS versions 14.5 and later. Older apps need to be reinstalled with a signature that is compatible with the new encryption keys. Normally this is done automatically but sometimes you need to update the app manually. This is mostly the case for users that bought Stage Traxx with another AppleID than the one they are running currently on the iPad.
Follow these steps to repair Stage Traxx version 2:
Make sure you are logged in to the App Store with the AppleID you used to purchase Stage Traxx. If you see Stage Traxx in the list of your purchased apps, then you are logged in with the correct AppleID. Now go to iPad Settings -> General -> iPad Storage. Select Stage Traxx and tap on Offload App. This will delete the app but keep your data.
Now open the app again it will be reinstalled from the App Store while your data stays intact.