When I opened the App, I received this message, 'Unable to connect to server', what can I do?

Symptoms

When I opened the App, I received this message, 'Unable to connect to server', what can I do?

Resolution

Please check the internet connection on your phone. If your phone is connected to WiFi, try switching to 3/4G and different WiFi's. Restart the App or restart your phone to try again. 

Causes

1. Poor internet connection on the phone (most common);
2. Server breakdown (very rare).