Hi, When i run "pkcon refresh" in a console, i haven't error message, the command works. For the test, i run "gnome-packagekit-data" before the command-line and there was the same problem. After to launch the command, i have now the updates's list in "gnome-packagekit-data".
This is the output console after to launch "pkcon refresh" : http://pastebin.com/NstLDk3f