Comment 4 for bug 1492889

the bug is also present in Ubuntu 15.04

the cause is that no crypttab is created

a workaround is as follows (using encrypted volume with existing btrfs on sda4 and boot on sda3 as an example):

(boot install disc, try Ubuntu without installing, and open the encrypted volume as required by https://bugs.launchpad.net/bugs/1066480 https://wiki.ubuntu.com/TrustyTahr/ReleaseNotes/#Boot.2C_installation_and_post-install)
# cryptsetup luksOpen /dev/sda4 sda4_crypt
(perform the installation)
# mount /dev/mapper/sda4_crypt /mnt
# cd /mnt
(clean up leakage)
# rm ubiquity-apt-clone/apt-clone-state-ubuntu.tar.gz
# rmdir ubiquity-apt-clone/ var/lib/ var/
(find the uuid)
# blkid /dev/sda4
(create missing crypttab using uuid)
# echo "sda4_crypt UUID=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx none luks" > /mnt/@/etc/crypttab
(update initrd and grub)
# cd @
# mount /dev/sda3 boot
# for d in dev sys proc; do mount --bind /$d $d; done
# chroot /mnt/@
# update-initramfs -k all -c
# shutdown -r now