@swlab That problem seems weird to me. I already saw a problem like:
Application Error - The connection to the server was unsuccessful. (file:///android_asset/www/index.html)
It already happened to me, and the solution was to increase the timeout, like in:
But in your case it tries to connect to http://localhost:8080/.
How is your package installed in your device? You said you run:
ionic cordova build android
That builds the package, but how do you deploy it to your device? Do you connect your Android device via USB and run:
cordova run android --nobuild
so that the app is deployed to your device? If not, try to do that.