Demo crashes on start

What happens when you disable your integrated sound card and connect your audio interface?

Please see this option, too: “Allow applications to take exclusive control of this device”
Link: Inexplicable audio dropouts/popping sounds only in Gig Performer 4 - #12 by npudar

If you still are unable to change the buffer size, can you try with the ASIO4ALL, the latest beta (beta 2)