How to fix "End of stream on null" network issue?

Some users of my Android application are frequently running into a network error with the following error message: "Unexpected end of stream on null".

I didn't find any clear way to solve it by looking at other similar question. I haven't been able to reproduce the network error on my end.

1) I have unsuccessfully appended Connection=close to the requests headers as this answer suggests

2) I have unsucessflly added .retryOnConnectionFailure(true) as this answer suggests

3) I have unsuccessfully searched for a server side issue as this answer suggests but the requests with network issues are not appearing in the nginx access.log file

Here is how I init Retrofit

OkHttpClient okHttpClient = new OkHttpClient().newBuilder()
                .connectTimeout(40, TimeUnit.SECONDS)
                .readTimeout(30, TimeUnit.SECONDS)
                .writeTimeout(30, TimeUnit.SECONDS)
                .retryOnConnectionFailure(true)
                .build();

Retrofit.Builder retrofitBuilder = new Retrofit.Builder()
                .baseUrl(BuildConfig.BASE_URL)
                .addConverterFactory(GsonConverterFactory.create())
                .client(okHttpClient);

this.retrofit = retrofitBuilder.build();

Here is my endpoint definition (in IUserService.java)

@POST("/v2/android/login/phone")
@Headers("Content-Type: application/json;charset=UTF-8;Connection=close")
Call<APIResult<Session>> phoneLogin(@Body APIRequest request);

And here is how the endpoint is queried

userService = retrofit.create(IUserService.class);
Call<APIResult<Session>> call = userService.phoneLogin(request);
call.enqueue(new GenericCallback<>(context, callback));

Our configuration:

Android (Java)

Retrofit 2.3.0: 'com.squareup.retrofit2:retrofit:2.3.0'

Okhttp 3.12.1: 'com.squareup.okhttp3:okhttp:3.12.1'

Server running on AWS Elastic Beanstalk, platform: Puma with Ruby 2.6 running on 64bit Amazon Linux/2.9.0

Can anyone explain better where the problem is coming from? Any idea how to reproduce this issue locally? Any way to solve this issue?

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