Similar thing happened to me whilst attempting to book Madrid to Sevilla flights earlier this year. I had to use a VPN reroute via the United States and book via that page. The Italian one did not work; I received similar errors.

The check-in desk agent actually raised a ruckus also because I checked in with my Italian ID card, when it was an American card I had used to purchase. I dug out my US passport and a supervisor ended up using my Italian passport but a unique experience nonetheless for me...along with the plane seats designed for midgets they used on the (thankfully) short flight but I digress. Maybe it is bad luck but IB always gives me issues. yet Alitalia doesn't LOL so YMMV as always.