Magento – Magento 2 Custom error Landing pages (500, 401 and etc)

you can customize (404) page from admin

Content->Pages->404 Not Found->Edit->content

here you can change whatever content you need.

If you look programmatically solution, add the below content in your <root-magent>/app/code/Vendor/Module/etc/di.xml

<type name="Magento\Framework\App\Router\NoRouteHandlerList">
<arguments>
    <argument name="handlerClassesList" xsi:type="array">
        <item name="default" xsi:type="array">
            <item name="class"
            xsi:type="string">Vendor\Module\Controller\NoRouteHandler</ite
            m>
            <item name="sortOrder" xsi:type="string">200</item>
        </item>
    </argument>
</arguments>
</type>

add this content in above mentioned controller

<?php
namespace Vendor\Module\Controller;
class NoRouteHandler implements \Magento\Framework\App\Router\NoRouteHandlerInterface {

    public function process(\Magento\Framework\App\RequestInterface $request) {
        $moduleName= 'cms';
        $controllerName = 'index';
        $actionName= 'index';

        $request
        ->setModuleName($moduleName)
        ->setControllerName($controllerName)
        ->setActionName($actionName);
        return true;
    }
}

In this controller you can set whatever module/controller/actionname for 404 page.

follow this link to set 401,403,500 page

I don't know exactly HOW it happened, but in my scenario the core_config_data table was corrupted. It had added a new row somehow with path set to general and value set to NULL:

This caused Magento\Framework\App\Config\ScopeConfigInterface::getValue() on public/vendor/magento/framework/Stdlib/DateTime/Timezone.php::scopeDate() to return NULL instead of Europe/Amsterdam. Needless to say, when the \DateTimeZone() object was created it immediately threw this exception.

So if anyone else encounters this error, your configuration might be an obvious place to start looking.