Cowboy shows error 400 after Phoenix successfully returning 200 for the same request_id

Hi all, maybe someone have similar experience or some ideas. We are using SMS service provider to send messages from our Elixir/Phoenix app. On every message sent this provider makes a call-back (POST request) over a specified api route in our application. After processing this call-back successfully we return 200 to the caller but just after this Cowboy show 400 error for the same request ID:

When this call black is made over HTTP everything is working flawlessly but after we switched to HTTPS - we have this error almost on every call. App is deployed on AWS and HTTPS requests are terminated on nginx

Are you able to supply any code?