"Failed installing image" after entering root password, loopback devices stay mounted

[Peter Schüller]

While running usb-creator-gtk it asks for root authentication to install the boot loader ("System policy prevents installation of the boot loader") but even if I enter the correct authentication the program gives a failure message and terminates. Afterwards two loopback devices stay mounted. I tried with and without the casper partition, both fails. also running usb-creator-gtk with sudo fails (without asking for root password).

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: usb-creator-gtk 0.2.56.3ubuntu0.2
ProcVersionSignature: Ubuntu 3.13.0-62.101-generic 3.13.11-ckt24
Uname: Linux 3.13.0-62-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.14.1-0ubuntu3.12
Architecture: amd64
CurrentDesktop: Unity
Date: Fri Aug 14 13:46:52 2015
InstallationDate: Installed on 2015-02-20 (174 days ago)
InstallationMedia: Ubuntu 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.2)
SourcePackage: usb-creator
UDisksDump: Error: [Errno 2] No such file or directory: 'udisks'
UpgradeStatus: No upgrade log present (probably fresh install)