MultiPart requests answer 502 despite server answer is 200

I have a python based server (aiohttp) waiting for rest requests, and Kong as front end for managing requests. The client side is WEB application built with Vue (The problem is the same if I use POSTman to send multipart requests).

Everything works perfect for every transactions except for MultiPart. Especfically my request has 2 parts, one is json (converted to text) and the other one is the attachments. The request actually gets the python server code, but repeated 6 times (standard Kong configuration 5 retries) in less than a second, and Kong is answering 502 once to the client webapp or postman despite server answer is 200 to every iteration.

Any tip or help will be very welcome.