Android 4 issue with Angular HttpClient

Anyone had any issues with the Angular HttpClient on Android 4.4 devices?

We have upgraded our app so it now uses Angular’s HttpClient rather than the old Http one. Everyything works fine across all devices (iOS and Android) apart from Android 4. When the app makes a Post request to the server on an Android 4.4 device an error is generated:

Http failure response for (unknown url): 599 undefined

Anyone had anything similar on Android 4 ?

I fixed this problem - the fix I needed is shown here