Unable to save video edits on Android

[coolmagic]

Hi. I've just started out with my Spark, and shot a few short videos to try it out. I've used the editor in the Go4 app, and saved the video. When I look in the production folder created by the app on my phone, it shows a grey box with an exclamation mark and states video not found. I'm using a Samsung Galaxy S7, and all apps and software are updated and current. Any ideas gratefully received.