Hello Sandor,
I am using booking.com. Indeed, it was not possible to book more than 12 months before. But in may 2016, I search for booking in june 2017 for Kumlbach last year, and it was possible; then I made a booking of 10 weeks until August 2017 for a room, then after I change the date, reducing the period only to August ... and the booking was so registered !!!. It is called 'going around the system', something that multinational companies and banks are very familar with to escape taxes on profits.
I ignore if six months earlier is possible, but perhaps direct contact with the Hotel woud be possible , because even 13 months before, the places in Kulmbach town was very small. I booked at 12 km distance...