Good news and bad news:
Good news: I received a couple of other crashlogs tonight and I can see why this is happening. I can reproduce it by enabling "Combine Headphone & USB output" and then exiting the app. Now when I try to start the app again, it will crash. It was easy to fix and I will send an updated version 3.4.10 today to Apple.
Bad news: There is no way to disable that parameter when the app crashes right after the start. So you will have to wait for version 3.4.10 to be released (hopefully tomorrow). However there will be a new beta version available with this fix in about an hour. So if you need Stage Traxx tonight, you can use the beta version available here: https://testflight.apple.com/join/pq4xbzSq