This is a message from the audio engine. It fails to start after switching back to Stage Traxx. Do you use other audio apps when ST3 is in the background? Might be related to that. I suspect another audio app is blocking the audio resources of the iPad because it is not shutdown yet.
It is no biggie, as ST3 also tries to restart the engine when you hit play so this fallback is working for you. Maybe I should just remove this error window and only show the error when restarting fails when hitting play. Because in that case you will notice that playback does not work.