Software updater spontaneously pops up request for bug report

[Quigi]

Running Trusty Tahr (14.04 LTS), after one of update-manager's periodic runs, it (or Apport?) popped up a dialog window.
Name: "Software Updater"
Title: "Could not initialize the package information"
Message:
   An unresolvable problem occurred while initializing the package information.

   Please report this bug against the 'update-manager' package and include the following error message:
   E:Opening /etc/apt/sources.list.d/canonical.list - ifstream::ifstream (13: Permission denied)

======
Herewith reported.

The file exists, but is not readable by other:
   -rw-rw---- 1 root root 59 Jun 24 08:05 /etc/apt/sources.list.d/canonical.list

This is not a distribution or release upgrade; /var/log/dist-upgrade/ is empty.

The issue seems to confuse Apport; I tried "ubuntu-bug update-manager" and "ubuntu-bug -w", but both pop up this message:
   The problem cannot be reported:

   E:Opening /etc/apt/sources.list.d/canonical.list - ifstream::ifstream (13: Permission denied)"

Maybe that file should be world readable for the benefit of ubuntu-bug? Initially update-manager had asked for my password in order to make changes to the system (and I have sudo).