Problem
initramfs-tools is only “half-configured”:
root@ucs:/boot# dpkg -l | grep initramfs
iF  initramfs-tools                                     0.130                                                               all          generic modular initramfs generator (automation)
Solution
Make sure the file /lib/systemd/systemd-udevdexists by re-installing if it’s missing:
apt-get --reinstall install udev