The solution for this issue is:
I added the following line to all files under the folder /var/www/nextcloud/apps/user_ldap/ajax/*:
require_once($_SERVER['DOCUMENT_ROOT'].'/lib/base.php');
A example for the file wizard.php:
* @license AGPL-3.0
*
* This code is free software: you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License, version 3,
* as published by the Free Software Foundation.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU Affero General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License, version 3,
* along with this program. If not, see <http://www.gnu.org/licenses/>
*
*/
require_once($_SERVER['DOCUMENT_ROOT'].'/lib/base.php');
// Check user and app status
\OC_JSON::checkAdminUser();
\OC_JSON::checkAppEnabled('user_ldap');
\OC_JSON::callCheck();