Unfortunately this is pretty easy to reproduce. I installed Ubuntu Server 15.10, set up an additional partition and formatted it with LUKS, and added to crypttab. That is all it takes to make the boot process hang while it asks for a password.
It appears that if the partition is unlocked in the initramfs, then it works. The initramfs cryptsetup is ok. But if initramfs doesn't unlock it - i.e. it's not the boot drive - then it's up to systemd to do so, which doesn't work.
Oddly enough, if I reboot then I can momentarily see the password prompt just before the system reboots. Of course by then it's too late. :(
Unfortunately this is pretty easy to reproduce. I installed Ubuntu Server 15.10, set up an additional partition and formatted it with LUKS, and added to crypttab. That is all it takes to make the boot process hang while it asks for a password.
It appears that if the partition is unlocked in the initramfs, then it works. The initramfs cryptsetup is ok. But if initramfs doesn't unlock it - i.e. it's not the boot drive - then it's up to systemd to do so, which doesn't work.
Oddly enough, if I reboot then I can momentarily see the password prompt just before the system reboots. Of course by then it's too late. :(