“Lost password” page triggers 404

Your theme or plugins are most likely modifying the “Lost password” link on your wp-login.php via the lostpassword_url filter.

How do I reset the “change password” page to the WP default one?

One would need to remove those filters callbacks.

Here’s one (untested) suggestion:

add_filter( 'lostpassword_url', function( $url, $redirect )
{
    remove_all_filters( 'lostpassword_url' );

    return wp_lostpassword_url( $redirect );
}, PHP_INT_MAX, 2 );

Otherwise one could search the install for add_filter( 'lostpassword_url' or peek into the $wp_filter global array. Here’s an example:

add_action( 'login_footer', function() use ( &$wp_filter )
{
    if( isset( $wp_filter['lostpassword_url'] ) )
        printf( '<!--%s-->', print_r( $wp_filter['lostpassword_url'], 1 ) );
});

that should print out the information as an HTML comment on the login page.