I am having the same issue. I'm staying at Newport Bay in May and am unable to see any dining reservations beyond the 2 month mark, despite it being all over the website etc that Disney hotel guests can book 12 months out. I tried googling around and got no helpful answers. People say you need to tap on the date and it asks you to sign in, but I can't even scroll beyond January without the error message popping up.
I booked through Attraction Tickets and they told me I should be able to do it, and offered no help when I said I couldn't.