In PHP I set a cookie using
setcookie($name, null, -1, '/');

However I get the following warning

Warning: Cannot modify header information - headers already sent by (output started at php://input:1)

which is even more strange, because I have ob_start() at the beginning of my PHP script.
I also set auto_prepend_file=NULL in my php.ini as described in Where could PHP output which started at php://input:1 be originating from?.

I use php-fpm 8.3 inside a Docker container, which is proxied by nginx.
As I understand it, php://input is a read only input stream, so where could the output be comming from?

Edit:
display_errors and error_reporting is off. php://input contains the data from the post request

csrf-token=abc&user=def+&password=ghi&stayloggedin=on

It also seems that the php://input is sent to the browser. Before the error occurs the content of php://input is displayed on a blank page in the browser, the user is then redirected. Does anyone have any idea on where to start searching?