Please change the unserialize function in vendor/magento/framework/Serialize/Serializer/Json.php.

You also need to check whether the string is serialized or not.

public function unserialize($string)
{
    /************** Custom code ***********/
    if($this->is_serialized($string))
    {
        $string = $this->serialize($string);
    }
    /************** Custom code ***********/
    $result = json_decode($string, true);
    if (json_last_error() !== JSON_ERROR_NONE) {
         throw new InvalidArgumentException('Unable to unserialize value.');

    }
    return $result;
}