Same issue here. So it's relevant.

Well, there's not much information to help you I am afraid. We will probably need a small example application that allows to debug.

@THenkeDE Some questions we could minimze the scope of the problem?

• Are you using EasyAdminBundle? (Because we are using, so maybe the issue is here)
• Are you setting custom cookies?

Same issue here, it happens when streaming a PDF (using DOMPDF) to the browser
Warning: Cannot modify header information - headers already sent by (output started at /app/vendor/dompdf/dompdf/src/Adapter/CPDF.php:982)

Output started at : https://github.com/dompdf/dompdf/blob/master/src/Adapter/CPDF.php#L982
Warning popping at : https://github.com/symfony/symfony/blob/7.4/src/Symfony/Component/HttpFoundation/Session/SessionUtils.php#L52

EDIT: I just changed to the Symfony way, ie: sending a binary response instead of using directly the "stream" method of the DOMPDF lib which is outputing content, causing the error with the kernel response AbstractSessionListener.

We'd need a reproducer to understand the issue.
To anyone seeing the same problem, please create a small app that one could run to debug this.
Skip "me too" comments unless you post a reproducer please :)

PHP: 8.3, Symfony: 7.3.0

I have exceptions in my logs which look very similar:

{"message":"Uncaught PHP Exception ErrorException: \"Warning: Cannot modify header information - headers
already sent by (output started at /var/www/html/src/App/Controller/ShowLocalFileController.php:29)\" 
at Response.php line 322","context":{"exception":{"class":"ErrorException","message":
"Warning: Cannot modify header information - headers already sent by (output started at 
/var/www/html/src/App/Controller/ShowLocalFileController.php:29)","code":0,"file":
"/var/www/html/vendor/symfony/http-foundation/Response.php:322"}},
"level":500,"level_name":"CRITICAL","channel":"request","datetime":"2025-06-04T11:56:38.706342+00:00",
"extra":{}}

The related code part is:

// code part simplified to demonstrate the case

    #[Route('/', name: 'home')]
    public function handleRequest(): Response
    {
        $filepath = ROOT_PATH.'mbl-2025-7.pdf';

        // load file and show in browser
        header('Content-type:'. mime_content_type($filepath));
        echo $this->filesystem->readFile($filepath);  // <======== line 29, is mentioned in the error log

        return new Response();
    }

Here is a reproducer based on the code of an proprietary app, so I hope it matches the case of the author and I extracted only the relevant code parts.

https://github.com/k00ni/cannot-modify-header-reproducer

I can provider further details if they are needed. The PDF file inside the repository is public domain by German law, in case that is relevant.

Workaround?

I solved it by replacing the mentioned code part with Symfony's BinaryFileResponse. So the code part has to be changed to:

// code part simplified to demonstrate the case

    #[Route('/', name: 'home')]
    public function handleRequest(): Response
    {
        $filepath = ROOT_PATH.'mbl-2025-7.pdf';

        $response = new \Symfony\Component\HttpFoundation\BinaryFileResponse($filepath);
        return $response;
    }

Symfony documentation provides further information about this class: https://symfony.com/doc/current/components/http_foundation.html#serving-files

Thanks for the reproducer. Actually, the workaround you posted is a fix. The previous code was just broken and worked by chance.

Let me close.
Note that there's #60603 with some related follow up, but linked to a recent change in 7.3, which is what you might be experiencing @k00ni