Regression: Cannot install signatures in Utopic any more

In the script /usr/share/system-config-printer/install-printerdriver.py I use package kit to add a repository to install printer driver packages, install the signature for te repository and finally install the driver package from the repository, to get a secure installation and the system's update facility also updating my printer driver package if an update gets available.

Under Trusty this works correctly, but under Utopic it stopped working.

Under Utopic I get the following if I run the Python statements of the script one by one at the Python prompt:

----------
till@till-twist:~$ python
Python 2.7.7 (default, Jun 4 2014, 05:21:28)
[GCC 4.8.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from gi.repository import GLib, PackageKitGlib
>>> import sys
>>> def progress(progress, type, user_data):
... pass
...
>>> repo_gpg_id = "E5220FB7014D0FBDA50DFC2BE5E86C008AA65D56"
>>> pk = PackageKitGlib.Client()
>>> res = pk.install_signature(PackageKitGlib.SigTypeEnum.GPG, repo_gpg_id, '', None, progress, None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
GLib.Error: g-dbus-error-quark: Error calling StartServiceByName for org.freedesktop.PackageKit: GDBus.Error:org.freedesktop.DBus.Error.Spawn.ChildExited: Launch helper exited with unknown return code 1 (25)
>>>
----------

The signature key is from Epson. I also tried with the key

F8897B6F00075648E248B7EC24CBF5474CFD1E2F

from OpenPrinting and got the same result.