Comment 39 for bug 1499746

the key made with the startup disk creator does not work, even on old hardware. nevertheless, simply copying the whole iso image on a key works.

thus, workaround (as root or sudo):

dd if=ubuntu-15.10-desktop-amd64.iso of=/dev/sdX bs=16M ; sync

replace "X" with your usb key letter, see eg dmesg or blkid, end then eject the drive. for "X" do not include any partition numbers.