FATAL error on hitting Rails API

Hi,

I am beginner in ruby/rails.
I have a rails api which when curled directly works as excpected.

But, when invoked through vent(wrapper on kafka queue) returns 4xx error. I have verified that the payload is correct and the api works perfectly fine when curled directly using the same paload.

I get the following default log from Rails logger:
Started POST "/" for at

Next log is expected to be something like:
Processing by # as JSON

But I do not get the second log, instead I see a log with empty message and severity = FATAL.

How do I debug this? I would like to understand when/where these default messages are logged.

Seems like the place holders where scrubbed.
Logs are as follows:

Started POST "/items" for 10.20.450.160 at 2019-01-07 09:20:11
Processing by ItemsController#create as JSON