I've made much more expensive tests and each time I found the same issue in IPN ERROR log:
result: ERROR : Invalid Seller Email: Ferry devant îlot [CProductInfoSub]
where "Ferry devant îlot" is the "item_name".

As I told you, this happens only with paypal real trasaction, never with simulator. I have no idea what could be the origin of this strange behaviour.
There is a lot of information on IPN ERROR report, hope these details, together with your help, could resolve this frustrating situation.

>result: ERROR : Invalid Seller Email: Ferry devant îlot [CProductInfoSub]
>where "Ferry devant îlot" is the "item_name".

Maybe you changed the value of "bussiness"?

<input type="hidden" name="business" value="....">

The email address you put in "business" must be the same as SIM_SELLER_EMAIL in web/config/simg/config.sim.inc.php. If they are different, you get the error message.