Just to make sure I understand you:
You are using Paypal Payments Standard?
You get to the confirm page on your site and confirm your order which brings you to paypal's site. On Paypal's site you click the continue shopping button and get an error?
What is the exect text of the error?
On the confirm page, look for the following hidden form elements:
<input type="hidden" name="cancel_return" id="cancel_return" value="http://www.yourdomein.com/cancel_checkout.php" />
<input type="hidden" name="return" id="return" value="http://www.yourdomein.com/checkout_success.php" />
and add another hidden form element:
<input type="hidden" name="shopping_url" id="return" value="http://www.yourdomein.com/cart.php" />
