By default, PHP accepts only a limited number of input variables. When that limit is exceeded, additional form fields are discarded before Magento receives them. This is common with extensive extensions and configuration screens.

You can inspect the current value using:

php -i | grep max_input_vars

A typical value is:

max_input_vars => 1000 => 1000

You can increase the limit in the relevant php.ini :

max_input_vars = 5000

Reload the PHP service afterwards. Note that the web server may use a different PHP configuration from the CLI. A correct CLI value does not guarantee that PHP-FPM uses the same one.

If Magento seems to save a configuration form but individual values vanish afterwards, inspect max_input_varsbefore searching for a bug in Magento itself.

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Acl/etc/acl.xsd">
    <acl>
        <resources>
            <resource id="Magento_Backend::admin">
                <resource id="Magento_Backend::stores">
                    <resource id="Vendor_Whatever::config" title="Foobar" sortOrder="90" />
                </resource>
            </resource>
        </resources>
    </acl>
</config>

In this example, the title is “Foobar”. This resolves the message and allows you to use the Magento 2 admin area normally.

php bin/magento admin:user:create --admin-user='nutzername' --admin-password='passwort123' --admin-email='nutzer@example.com' --admin-firstname='Ihr' --admin-lastname='Nutzer'

After running the command (make sure to change the sample username and password first), you should be able to log in again. You can then remove the old user or assign a new password.

Undefined array key 1234 in vendor/magento/module-catalog/Ui/DataProvider/Product/Form/Modifier/Eav.php on line 604

The ID can vary, but the cause is the same. Rebuilding the indexes with the following command, executed from your Magento 2 root directory, will often help:

php bin/magento indexer:reindex

You should now be able to open products in the admin area again.

php bin/magento config:set system/security/max_session_size_admin 0

php bin/magento config:set system/security/max_session_size_admin 0

That should resolve the issue.

This should resolve the issue.

UPDATE `admin_user` SET `password` = MD5('NEUESPASSWORT') WHERE `username` = 'ADMINUSERNAME';

<admin>
    <routers>
        <adminhtml>
            <args>
                <frontName>your-new-backend-route</frontName>
            </args>
        </adminhtml>
    </routers>
</admin>

After adding that to your app/etc/local.xml file, clear the cache and check if your admin is now available at http://www.your-shop.com/your-new-backend-route instead of http://www.your-shop.com/admin.

To add a field and a comment, open your modules system.xml and add the below XML:

<some_thing translate="label">
    <label>Test</label>
    <frontend_type>text</frontend_type>
    <sort_order>0</sort_order>
    <show_in_default>1</show_in_default>
    <show_in_website>1</show_in_website>
    <show_in_store>1</show_in_store>
    <comment>Thats a comment!</comment>
<some_thing>