Reproduction steps:
1) Open "Notifications Client" project
2) Go to "Test" scene
3) Build and run project on Android (4.3 or earlier) device
4) Press "Register" button
-- Using ADB notice that after calling UnityWebRequest.Send(); "java.io.IOException: No authentication challenges found" is being thrown

Note:
"java.io.IOException: No authentication challenges found" is being thrown only when accessing server which returns 403 - forbidden.