Reviewing the source it looks to be stalling in partman/finish.d/apt_clone_save

I added 'set -x' to top of that file in the live system and captured the following in its syslog. Looks like there's a failure to create the root-fs directory structure and so there is no mountpoint for the /var/ file-system:

xubuntu@xubuntu:~$ ls -latr /tmp/tmp.GRbQsIC1FI/
total 20
drwx------  2 root root 16384 Apr  8 17:02 lost+found
drwxr-xr-x  3 root root  4096 Apr  8 17:02 .
drwxrwxrwt 20 root root   500 Apr  8 21:06 ..

--- syslog ---
Apr  8 20:01:02 xubuntu ubiquity: + db_progress START 0
Apr  8 20:01:02 xubuntu ubiquity:  1 ubiquity/install/title
Apr  8 20:01:02 xubuntu ubiquity: + _db_cmd PROGRESS START
Apr  8 20:01:02 xubuntu ubiquity:  0 1 ubiquity/install/title
Apr  8 20:01:02 xubuntu ubiquity: + _db_internal_IFS= #011
Apr  8 20:01:02 xubuntu ubiquity: 
Apr  8 20:01:02 xubuntu ubiquity: + IFS= 
Apr  8 20:01:02 xubuntu ubiquity: + printf %s\n PROGRESS START 0 1 ubiquity/install/title
Apr  8 20:01:02 xubuntu ubiquity: + IFS=
Apr  8 20:01:02 xubuntu ubiquity: 
Apr  8 20:01:02 xubuntu ubiquity: + read -r _db_internal_line
Apr  8 20:01:02 xubuntu ubiquity: + IFS= #011
Apr  8 20:01:02 xubuntu ubiquity: 
Apr  8 20:01:02 xubuntu ubiquity: + RET=OK
Apr  8 20:01:02 xubuntu ubiquity: + return 0
Apr  8 20:01:02 xubuntu ubiquity: + db_progress INFO ubiquity/install/apt_clone_save
Apr  8 20:01:02 xubuntu ubiquity: + _db_cmd PROGRESS INFO ubiquity/install/apt_clone_save
Apr  8 20:01:02 xubuntu ubiquity: + _db_internal_IFS= #011
Apr  8 20:01:02 xubuntu ubiquity: 
Apr  8 20:01:02 xubuntu ubiquity: + IFS= 
Apr  8 20:01:02 xubuntu ubiquity: + printf %s\n PROGRESS INFO ubiquity/install/apt_clone_save
Apr  8 20:01:02 xubuntu ubiquity: + IFS=
Apr  8 20:01:02 xubuntu ubiquity: 
Apr  8 20:01:02 xubuntu ubiquity: + read -r _db_internal_line
Apr  8 20:01:02 xubuntu ubiquity: + IFS= #011
Apr  8 20:01:02 xubuntu ubiquity: 
Apr  8 20:01:02 xubuntu ubiquity: + RET=OK
Apr  8 20:01:02 xubuntu ubiquity: + return 0

Apr  8 20:01:02 xubuntu ubiquity: + mount /dev/mapper/ELLOE00-rootfs /tmp/tmp.GRbQsIC1FI
Apr  8 20:01:02 xubuntu ubiquity: + [ -n /dev/mapper/ELLOE00-var ]
Apr  8 20:01:02 xubuntu kernel: [ 3188.701770] EXT4-fs (dm-2): mounted filesystem with ordered data mode. Opts: (null)

Apr  8 20:01:02 xubuntu ubiquity: + mount /dev/mapper/ELLOE00-var /tmp/tmp.GRbQsIC1FI/var
Apr  8 20:01:02 xubuntu ubiquity: mount: /tmp/tmp.GRbQsIC1FI/var: mount point does not exist.

Apr  8 20:01:02 xubuntu ubiquity[24878]: debconffilter_done: ubiquity.components.partman_commit (current: ubi-timezone)
Apr  8 20:01:04 xubuntu ubiquity[24878]: switched to page timezone
Apr  8 20:01:06 xubuntu localechooser: info: debian-installer/language preseeded to 'en_GB:en' (seen: false)
Apr  8 20:01:06 xubuntu localechooser: info: debian-installer/country preseeded to 'GB' (seen: true)
Apr  8 20:01:06 xubuntu localechooser: info: debian-installer/locale preseeded to 'en_GB.UTF-8' (seen: true)
Apr  8 20:01:06 xubuntu localechooser: info: Preseeded language ignored: unknown language code
Apr  8 20:01:06 xubuntu localechooser: info: Language = 'en'
Apr  8 20:01:06 xubuntu localechooser: info: line=en;0;US;en_US.UTF-8;;console-setup
Apr  8 20:01:06 xubuntu localechooser: info: Set debian-installer/language = 'en'
Apr  8 20:01:06 xubuntu localechooser: info: Default country = 'US'
Apr  8 20:01:06 xubuntu localechooser: info: Default locale = 'en_US.UTF-8'
Apr  8 20:01:06 xubuntu localechooser: info: Set debian-installer/consoledisplay = 'console-setup'
Apr  8 20:01:06 xubuntu localechooser: info: Set debian-installer/country = 'GB'
Apr  8 20:01:07 xubuntu localechooser: info: Set debian-installer/locale = 'en_GB.UTF-8'
Apr  8 20:01:07 xubuntu localechooser: info: System locale (debian-installer/locale) = 'en_GB.UTF-8'
Apr  8 20:01:07 xubuntu localechooser: info: Set debian-installer/language = 'en_GB:en'
Apr  8 20:01:07 xubuntu ubiquity[24878]: Not installing yet, partitioned: False, timezone_set True
Apr  8 20:01:07 xubuntu ubiquity[24878]: debconffilter_done: ubi-timezone (current: ubi-timezone)
Apr  8 20:01:07 xubuntu ubiquity[24878]: Step_before = stepLocation
Apr  8 20:01:07 xubuntu ubiquity[24878]: switched to page usersetup
Apr  8 20:01:28 xubuntu ubiquity[24878]: debconffilter_done: ubi-usersetup (current: ubi-usersetup)
Apr  8 20:01:28 xubuntu ubiquity[24878]: Step_before = stepUserInfo