The XXX plugin has been deactivated due to an error: The plugin does not have a valid header

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The active_plugins data is incorrect. Here’s a sample from one of my test sites:

a:3:{i:0;s:34:"advanced-custom-fields-pro/acf.php";i:4;s:31:"code-snippets/code-snippets.php";i:21;s:24:"wordpress-seo/wp-seo.php";}

Or:

Array
(
    [0] => advanced-custom-fields-pro/acf.php
    [4] => code-snippets/code-snippets.php
    [21] => wordpress-seo/wp-seo.php
)

Notice that the plugins are referred to by the path to the main plugin file, relative to the plugins folder. The main plugin folder is what contains the plugin header (example from here):

/**
 * Plugin Name:       My Basics Plugin
 * Plugin URI:        https://example.com/plugins/the-basics/
 * Description:       Handle the basics with this plugin.
 * Version:           1.10.3
 * Requires at least: 5.2
 * Requires PHP:      7.2
 * Author:            John Smith
 * Author URI:        https://author.example.com/
 * License:           GPL v2 or later
 * License URI:       https://www.gnu.org/licenses/gpl-2.0.html
 * Text Domain:       my-basics-plugin
 * Domain Path:       /languages
 */

Since /wp-content/plugins/classic-editor is a directory, not a file, it doesn’t have a plugin header, so gets deactivated.