aristeides, in, provided a database where when you add to the cart, you get the message
EntityMetadataWrapperException: Invalid data value given. Be sure it matches the required data type and format. in EntityMetadataWrapper->set() (line 110 of /home/rfay/workspace/commerce/sites/all/modules/entity/includes/entity.wrapper.inc).
I note that this warning is displayed when visiting the product line item type configuration page at admin/commerce/config/line-items/product/fields, even though the product reference module is in fact enabled:
Inactive fields are not shown unless their providing modules are enabled. The following fields are not enabled:
Product (commerce_product) field requires the Autocomplete text field widget provided by commerce_product_reference module
@aristeides, if you would give us information on when you created this test site, how you created it, etc. that would be great.