It looks like, after the Gateway Timeout error, participant records are increasing at about 10-50/second. This might just be a UX issue, and not a backend problem since all the records are still being created, just not instantly after clicking "upload".

The upload button should be disabled during the uploading of CSV imports.

:+1: easy to do

And including some user feedback like a progress bar, and a mechanism to keep the connection alive would be ideal.

About keep connection : more complex but there are solution using ajax and https://www.php.net/manual/fr/function.ob-flush.php and https://www.php.net/manual/fr/function.flush.php
But if we create something : we create it for more upload (VV for example)

"The gateway did not receive a timely response from the upstream server or application." is a problem of your webserver configuration.
Your webserver has a shorter timeout than the PHP-FPM process, that's why it keeps working in the background. You need to raise the timeout on the webserver here.

@mhladun : see https://gitlab.com/SondagesPro/TokenManagement/BigSizeTokenUpload

Allow you to separate the uploading file and import part: you can upload, then you can import.

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Capture d’écran du 2026-08-07 18-08-51.png (91,730 bytes)   

There is a separate and valid request in here for some kind of visual indication that the import is currently taking place, because these can take a while.
It should also give an alert that you should not navigate away.

As for DenisChenu's solution above - it probably solves a large part of the problem but the fact it's needed at all implies that the core itself needs to change to accommodate this user scenario. It is a workaround for something that should "just work".