Http request timeout, server-side?

Help!
1

Hi I’m using: SWI-Prolog version 8.3.18

I just setup a simple test server using library(http/http_server) from the very template given here.
I’m interested in handling POST request with files (10K < expected_size < 1M). I ran a test that worked like a charm with a 300-byte file, but when scaling to a 900K file I get error 500 with this message :

"message": "copy_stream_data/3: Timeout in read from <stream>(0x5588f9ed9900)"

My code is as simple as this:

:- use_module(library(http/http_server)).

:- initialization
    http_server([port(8080)]).

:- http_handler('/generate', gen_handler, []).

gen_handler(Request) :-
    stream_property(Stderr, file_no(2)),
    http_parameters(Request, [], [form_data(Data)]),  % this is where it gets blocked for any file bigger than ~150K
    forall(member(Key=Content, Data), format(Stderr, '~p~n~p~n~n', [Key, Content])).

I checked the client’s behaviour with a non Prolog server and it works.
I tried to increase the server-side timeout by passing an option to http_server, no result.

I have trouble debugging my server since it is multithreaded, but I wonder what could prevent data communication when the file is bigger.