Comment 12 for bug 1616368

I believe that I ran into the same issue while trying to run do-release-upgrade to upgrade my machine from 14.04.5 to 16.04.5.

It would seem that the problem is that the plymouth package does not properly depend on the udev package, so when both are being upgraded (as is done during do-release-upgrade), the postinst script for plymouth is run before the postinst script for udev. This causes the plymouth postinst to complain about the udev service not being enabled, which doesn't happen until after the udev postinst runs.

A log snippet from my do-release-upgrade run that shows udev postinst running after plymouth postinst:
```
Setting up plymouth (0.9.2-3ubuntu13.5) ...
update-initramfs: deferring update (trigger activated)
update-rc.d: warning: start and stop actions are no longer supported; falling back to defaults
insserv: Service udev has to be enabled to start service plymouth
insserv: exiting now!
update-rc.d: error: insserv rejected the script header
dpkg: error processing package plymouth (--configure):
 subprocess installed post-installation script returned error exit status 1
Setting up libpython-stdlib:amd64 (2.7.12-1~16.04) ...
Setting up gir1.2-glib-2.0:amd64 (1.46.0-3ubuntu1) ...
Setting up udev (229-4ubuntu21.15) ...
```

I can see from http://archive.ubuntu.com/ubuntu/pool/main/p/plymouth/plymouth_0.9.3-1ubuntu7.debian.tar.xz that the bionic version of plymouth does indeed have a dependency on udev. So perhaps this same dependency just needs to also be added to the xenial version?