I have a very similar issue. Upstart works but systemd does not. Lucs encrypted logical volume on a work computer. It has worked flawless with boot for many versions of Ubuntu (even before upstart I think). I can produce the syslog with two boots tomorrow. Also have taken a picture of the actual error message during boot with systemd, which is attached. Upstart presents the password prompt, systemd does not.
I have a very similar issue. Upstart works but systemd does not. Lucs encrypted logical volume on a work computer. It has worked flawless with boot for many versions of Ubuntu (even before upstart I think). I can produce the syslog with two boots tomorrow. Also have taken a picture of the actual error message during boot with systemd, which is attached. Upstart presents the password prompt, systemd does not.