Error 500 while accessing Vaadin app over 3G network

I have a Vaadin 6 app hosted on AWS which I can no longer access via my phone’s 3G network. It only stopped working on 3G like two weeks ago. However, everything works well while using the office’s Wi-Fi(fibre-based).

I tried updating to 6.8.16, recompiling the widgetset, clearing my browser cache but nothing works. See attached network activity screenshots as caprured from Google Chrome Inspector.

What could be the problem?