Yes I have read many people have about the same problem but not exactly. Let me explain:

Every time I click on 'create invoice' button in a Sale Order I get this message (in french as I'm french):

L'opération n'a pas pu être complétée, probablement à la suite d'une :
- suppression : vous essayez de supprimer un enregistrement auquel d'autres enregistrements font référence- création/modification : un champ requis n'est pas correctement rempli
[objet référencé : Product - product.product]

(I translate as

The operation could not be completed, probably after:
- deletion : you try to delete a record that other records are referencing
- create/modify : a reqired is not filled correctly
[referenced item : Product - product.product])

Thx if you could help avoid me to manually create my invoices in this cases (when they already ares done in a certain way).