I had the same situation, hope you can fix it. @joomla-cms-bot

I can't confirm this issue while using Joomla 4 core installation:

• Created file JoomlaRoot/error-404.html

Blaubbbbbbbb Balb Blabbbb

• Changed error.php of cassiopeia to

<?php
defined('_JEXEC') or die;

use Joomla\CMS\Factory;

$app = Factory::getApplication();
$lang = Factory::getLanguage();

header("HTTP/1.0 404 Not Found");

if ($this->error->getCode() == '404')
{
    switch ($lang->get('tag')){
        case 'de-DE':
            header('Location: /error-404.html');
            die();
        break;
case 'en-GB':
            header("Location: /error-404.html");
            die();
        break;
        default:
            header("Location: /error-404.html");
            die();
        break;
    }
}

• Entered a not existing URL in browser.
• and see Blaubbbbbbbb Balb Blabbbb in browser => redirection to error-404.html works.

hidden menu -> custom error page with sp-pagebuilder. in J3 there was no need to put a error.html in root, just redirect to a link from joomla menu. this routing dont work in j4.

Changed my test case:

• Created a Joomla menu item with alias error (first level menu item). Hidden in menus.

• Changed SEO settings in configuration
  

• Adapted my code in error.php Because you told us in german forum that your Joomla is inside a subfolder without custom domain now with Uri.

<?php
defined('_JEXEC') or die;

use Joomla\CMS\Factory;
use Joomla\CMS\Uri\Uri;

$app = Factory::getApplication();
$lang = Factory::getLanguage();

header("HTTP/1.0 404 Not Found");

if ($this->error->getCode() == '404')
{
	switch ($lang->get('tag')){
		case 'de-DE':
			header('Location: ' . Uri::root(true) . '/error.html');
			die();
		break;
			header('Location: ' . Uri::root(true) . '/error.html');
			echo 'en';
			die();
		break;
		default:
			header('Location: ' . Uri::root(true) . '/error.html');
			die();
		break;
	}
}

• redirection works as expected when I call an invalid url

• So this seems not to be a Joomla 4 core issue.

i changed the subfolder to root. so the page is direct accessible. unicode aliase -> on please.
error.html or error-404.html.. simply dont matter. also tried the real link: index.php?option...
pls check mail.

Which mail?

To get correct routing to menu item use Route::_('index.php?Itemid=111') where 111 is ID of your menu item.

Please ask more for support on a forum.
I closing this, it not looks like Joomla issue, but some problem with your code.

i asked on a forum thx. seems nobody can explain routing in j4. code is posted so everybody can modify and test my code :) thx for closing. its a joomla issue cause: blog detail works, blog page not. needs just time till someone find the bad routing.. i'll wait.