Cart66 WP Affiliate Platform Fatal Error

I installed WP Affiliate Platform and found it created an error message in my site’s sidebar where I have the Cart66 Lite shopping cart widget. The error message is:

Fatal error: Cart66CartWidget::widget() [cart66cartwidget.widget]: The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition “Cart66Cart” of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide a __autoload() function to load the class definition in /home/magmed/public_html/wp-content/plugins/cart66-lite/models/Cart66CartWidget.php on line 19

Deactivating the WP Affiliate Plugin gets rid of the message, but I want to be able to integrate the WP Aff plugin and still use the Cart66 widget.

Strangely, I installed both Cart66 Lite and WP Affiliate Platform on another test blog and there is no problem with the widget on that site (although I did not configure cart66 with the same settings I have on the first site).

I have the most recent versions of the plugins on both sites.
Any ideas what could be the problem and how to fix it?

In the error message it mentions line 19, which reads:

$this->_items = Cart66Session::get(‘Cart66Cart’)->getItems();

Is there anything obvious that needs changing in that line?

http://wordpress.org/extend/plugins/cart66-lite/