I had two products defined by shortkeys that worked fien. Then I added the HTML code for shopping cart on the same page. Then I got the following message and the menu diapeared:
Warning: Invalid argument supplied for foreach() in .../wordpress/wp-content/plugins/wordpress-simple-paypal-shopping-cart/wp_shopping_cart.php on line 92
I removed all wp-shopping-cart codes and shortkeys from the page, but the error is still there.