OK, so it happened when you selected a 25 minute long multitrack song? It seems that your iPad has trouble loading this file. Maybe too big or not enough main memory to buffer it. The iPad Mini 2 is a bit limited on main memory.
Anyway, to get Stage Traxx working again do the following to prevent it from trying to load this file when you open the app:
- Close Stage Traxx in the app switcher (double tap on home button)
- Open the iOS Files app and go to the Stage Traxx 3 folder
- Rename the audio file for the song you had selected. If it is a multitrack song this file will be in the Multitrack-Files folder
- Start Stage Traxx again
Now when Stage Traxx is working again we need to find out why this files does not work. Could you send the multitrack files for this song to support@stagetraxx.com? I have a iPad Air 1 with the same iOS version as you and I could try to reproduce the problem with your file(s).