When BASIC Auth is enabled on the server side and proper credentials entered in the Android App, the phone successfully connects to register the sensor and the result templates but no data is received by the server although the InsertResult request is initiated.
After looking at the packets in Wireshark it seems that the Auth header is not resent properly after receiving the initial 401 response when using a chunked transfer encoding. Need to check if it's a bug the okhttp client on Android or if it also happens in OpenJDK.