I was able to access the hosting panel now, but there are no error logs besides the error 500 on the site, following the update, with the message already cited "Composer detected issues in your platform: Your Composer dependencies require a PHP version ">= 8.4.1". You are running 8.3.30.".
But I activated Joomla Debug mode and I see the error coming from here: JROOT/plugins/system/helixultimate/vendor/composer/platform_check.php:22
which contains the following statement
if (!(PHP_VERSION_ID >= 80401)) {
$issues[] = 'Your Composer dependencies require a PHP version ">= 8.4.1". You are running ' . PHP_VERSION . '.';
}
The documentation of Helix ultimate states, though, that it should support PHP 8.3 as well