I have an error WordPress database error: [You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version

The problem here you are passing table name as a placeholder in prepare statement. Which is not allowed. You can directly add the table name in your query without quotes.
Your query do not have any placeholder and arguments. So No need for prepare statement.

Try following code.

global $wpdb;
$vragen = $wpdb->get_results( "SELECT * from questions ORDER BY id DESC", 
    ARRAY_A
);