Confirmed: having an existing LUKS or LVM install and trying to reinstall over it causes ubiquity to try to install grub to /dev/dm-0 instead of /dev/sda, which fails.
Confirmed: having an existing LUKS or LVM install and trying to reinstall over it causes ubiquity to try to install grub to /dev/dm-0 instead of /dev/sda, which fails.