In some cases, the authentication token of the mobile app will expire. That can cause the app to crash repeatedly (in most cases you will see an error message at the bottom of the app that reads "org.json.JSONException: No value for error").

To solve this, pull the settings menu from the left side of the app (drag your finger from the left edge of the device towards the center). Tap "Logout". This will take you to the login screen; a new authentication token will be assigned to you once again.