Comment 19 for bug 1597466

Also Ubuntu 18.04.1 LTS here.

I tracked down the issue to the plymouth-theme-kubuntu-text.postinst script:

Comparing /var/lib/dpkg/info/plymouth-theme-ubuntu-text.postinst and /var/lib/dpkg/info/plymouth-theme-kubuntu-text.postinst we see that the first has:

----------------------------------------
case "$1" in
    configure|triggered)
        if [ "$1" = "configure" ]; then
                # We remove leftover generated file from previous plymouth version. Can be removed on xenial+1
                if dpkg --compare-versions "$2" le-nl "0.9.2-3ubuntu1~"; then
                        rm -f /lib/plymouth/themes/ubuntu-text/ubuntu-text.plymouth
                fi
        fi
----------------------------------------

while the second has:

----------------------------------------
case "$1" in
    configure|triggered)
        # We remove leftover generated file from previous plymouth version. Can be removed on xenial+1
        if dpkg --compare-versions "$2" le-nl "1:16.04ubuntu1~"; then
            rm -f /lib/plymouth/themes/kubuntu-text/kubuntu-text.plymouth
        fi
----------------------------------------

Notice the missing check for the "configure" argument. Also see http://man7.org/linux/man-pages/man5/deb-postinst.5.html:

       postinst configure old-version
              After the package was installed.

       postinst triggered trigger-name...
              After the package was triggered.

The actual "bad syntax" message is then generated because
  dpkg --compare-versions "$2" le-nl "1:16.04ubuntu1~"
resolves to
  dpkg --compare-versions "/etc/lsb-release" le-nl "1:16.04ubuntu1~"
which is of course not what dpkg expects ("/etc/lsb-release" being the trigger-name)

So the bug seems to be a missing check for "configure" in the kubuntu version of the plymouth-theme postinst script.