Steps to reproduce the issue

When using PHP 8.4

When using joomla 6.0.2 i sudenly got the following error:

[08/Jan/2026:10:18:36 +0100] "GET / HTTP/2.0" 500 0 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/143.0.0.0 Safari/537.36 Edg/143.0.0.0"
2026/01/08 10:18:36 [error] 12136#12136: *12 FastCGI sent in stderr: "PHP message: PHP Fatal error:  Uncaught TypeError: iterator_count(): Argument #1 ($iterator) must be of type Traversable|array, Closure given in /var/www/website/libraries/vendor/paragonie/sodium_compat/autoload-php7.php:9
Stack trace:
#0 /var/www/website/libraries/vendor/paragonie/sodium_compat/autoload-php7.php(9): iterator_count()
#1 /var/www/website/libraries/vendor/paragonie/sodium_compat/autoload.php(41): require_once('...')
#2 /var/www/website/libraries/vendor/composer/autoload_real.php(41): require('...')
#3 /var/www/website/libraries/vendor/composer/autoload_real.php(45): {closure:ComposerAutoloaderInitf577684c10af21b55e694ebddac803ca::getLoader():37}()
#4 /var/www/website/libraries/vendor/autoload.php(25): ComposerAutoloaderInitf577684c10af21b55e694ebddac803ca::getLoader()
#5 /var/www/website/libraries/bootstrap.php(34): require('...')
#6 /var/www/website/includes/framework.php(17): require_once('...')
#7 /var/www/website/includes/app.php(29): require_" while reading response header from upstream, client: 2a02:22a0:eee3:cb00:469c:aa9c:365c:89f, server: oa-judo.nl, request: "GET / HTTP/2.0", upstream: "fastcgi://unix:/var/run/php/php8.4-fpm.sock:", host: "oa-judo.nl"

After some digging i had to do the following:

To make PHP 8.4 stable

1. Exclude autoload_psr4.php from OPcache (best practical fix)
   If you control PHP settings, add an OPcache blacklist file (Debian/Ubuntu paths shown; adapt to your system):

php.ini / opcache ini:
opcache.blacklist_filename=/etc/php/8.4/fpm/opcache-blacklist.txt

/etc/php/8.4/fpm/opcache-blacklist.txt
*/cache/autoload_psr4.php
*/administrator/cache/autoload_psr4.php

Reload php-fpm after changing this.
This prevents OPcache from caching exactly the file that Joomla regenerates and that seems to trigger your “first request 500”.

So now and then i still see the errors, so it is not 100% stable.