I've just upgraded our e-commerce website which uses Magento version 1.5. I upgraded it to the latest 188.8.131.52. Really lots and lots of bug fixes to be done and of them is what's the title of this post.
Anyways, you can fix it by going to Mage_Reports_Model_Resource_Product_Collection::setProductEntityId and remove the (int) because it's trying to convert the string into integer. This is not the best fix but I'm in a hurry so if you found something that is more appropriate then please leave a comment and don't just runaway. Cheers
public function setProductEntityId($entityId)
$this->_productEntityId = (int)$entityId;