If you are creating hybrid mobile app that is running in the sandox but not other environments like UAT,accp,prod etc, make sure that you have deployed your app to sandbox in the Mendix cloud first before trying to open the mobile app. If you try to open the app before doing this, you will receive the following error : Cannot initialize app in client side. And also make sure apk is reaching out to correct end point url or not.
You could use adb logcat to checkout for any errors on the OS level that might help you out. the debug process is similar for Native
If you are running Mendix on your local machine, it may help to enable “bundle widgets when running locally”
If this doesnt help, chances are that one of the widgets is causing this.
You could strip the application from any custom widgets one by one and see if that fixes it.