The ImportProduct Process initialized new Products with a Java Constructor which get the XX_I_Product as an argument. This constructor does not initialize volume and weight. (If you just update an old Product the error does not show up.)

While repairing it I found that in I_Product both the fields are integer fields and not BigDecimal. The database fields are numeric (this is right). This is because they are wrong declared in the AD.

Proposed Solution:

change the Fields' Reference in the AD to "Amount"
re-create Model Interface
write the missing initializers in the constructor