Impossible to set root password in Ubuntu Touch

[Borim]

On the bq Aquaris Ubuntu Edition it is not possible to set a root password. Normally you can use
sudo passwd root
to set the root password, but this fail with following message:
passwd: Authentication token manipulation error
passwd: password unchanged

On the Ubuntu on my pc I do not use a root password, as my user password is strong enough. But on a smart phone I use a shorter password, as I have to enter it way too often. Also I guess this counts for most other users.
The root access should be still secure, so a separate root password is needed.

ProblemType: Bug
DistroRelease: Ubuntu RTM 14.09
Package: passwd 1:4.1.5.1-1.1ubuntu2
Uname: Linux 3.4.67 armv7l
ApportVersion: 2.14.7-0ubuntu8
Architecture: armhf
Date: Sat Apr 18 18:30:19 2015
InstallationDate: Installed on 2015-04-10 (7 days ago)
InstallationMedia: Ubuntu Utopic Unicorn (development branch) - armhf (20150410-232623)
SourcePackage: shadow
UpgradeStatus: No upgrade log present (probably fresh install)
modified.conffile..etc.cron.daily.passwd: [deleted]

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[lgd (lgd)]

I could set the password (for a second user) by hacking /etc/shadow, deleting the password hash in field 2 between the delimiters ":" and copy the (hardened before) password hash from the user to there for example. But the user password is in /var/lib/extrausers/shadow.