hmm… there are known issues with the date plugins for users in some regions. I wonder if that might be the case here. @Roldy maybe you can confirm?
If that is the case, then you could create something similar using some date math. For example, you could use the following formula in a math column:
Year(Date)*12 + Month(Date) - where Date is your “data vendita”
That would give you a number, that would be the same for all records in the same month. So you could use that to build the relation.