This is the generic error that you get when something goes wrong but there is no proper error handling implemented. If this is a native app you might want to enable logging to pinpoint the cause by following the instructions on: https://docs.mendix.com/refguide/mobile/building-efficient-mobile-apps/logging/
If it is a online app that you open on your phone inspect the logs for the error that leads to the generic error that is shown to the user.