I am a new Nuki User from germany and I have similar problems / questions. Unfortunately, API calls to the bridge are very unstable in my environment. I actually try and test with Curl, Loxone Virtual HTTP Input and Chrome.
Sometimes I get a “HTTP 503 unreachable” or the connection is temporarily not possible and curl writes after a few seconds “Empty reply from server”. It is a mix of working API Calls, Error 503 and no or empty respond.
My hardware:
Nuki Bridge (ID 18xxxxxx) Firmware Version 2.2.12
Nuki Smart Lock 2.0 Firmware 2.5.4
The quality of the Wi-Fi and Bluetooth connection is good