Fatal error Joomla-PHP | Domain 1 & 1

I have a contracted domain in 1 & 1 and I've been on the web for a few days and I get this error . Do you know which can be the source of the problem ?

Thank you for your attention.

Fatal error: Cannot use Joomla\String\String as String because 'String' is a special class name in /homepages/43/d585417661/htdocs/libraries/vendor/joomla/registry/src/Format/Json.php on line 12

URL of the web: link

Json.php file code:

   <?php
/**
 * Part of the Joomla Framework Registry Package
 *
 * @copyright  Copyright (C) 2005 - 2015 Open Source Matters, Inc. All rights reserved.
 * @license    GNU General Public License version 2 or later; see LICENSE
 */

namespace Joomla\Registry\Format;

use Joomla\Registry\AbstractRegistryFormat;
use Joomla\String\String;

/**
 * JSON format handler for Registry.
 *
 * @since  1.0
 */
class Json extends AbstractRegistryFormat
{
    /**
     * Converts an object into a JSON formatted string.
     *
     * @param   object  $object   Data source object.
     * @param   array   $options  Options used by the formatter.
     *
     * @return  string  JSON formatted string.
     *
     * @since   1.0
     */
    public function objectToString($object, $options = array())
    {
        return String::unicode_to_utf8(json_encode($object));
    }

    /**
     * Parse a JSON formatted string and convert it into an object.
     *
     * If the string is not in JSON format, this method will attempt to parse it as INI format.
     *
     * @param   string  $data     JSON formatted string to convert.
     * @param   array   $options  Options used by the formatter.
     *
     * @return  object   Data object.
     *
     * @since   1.0
     */
    public function stringToObject($data, array $options = array('processSections' => false))
    {
        $data = trim($data);

        if ((substr($data, 0, 1) != '{') && (substr($data, -1, 1) != '}'))
        {
            $ini = AbstractRegistryFormat::getInstance('Ini');
            $obj = $ini->stringToObject($data, $options);
        }
        else
        {
            $obj = json_decode($data);
        }

        return $obj;
        }
    }

UPDATED

I'm trying to update the Joomla version, from 3.4.3 to 3.6.4 and Skip the following Error :