Can't add Ubuntu as system role in Mythbuntu control center

Installed Mythbuntu 14.04.2 LTS. In Control Centre, Sytem Roles, selected Ubuntu Desktop as Desktop Role & Apply. The confirmation dialog only displays one of two lines of the proposed action. Selecting Apply and entering password & selecting Authenticate brings up a dialog that says:

"CD/DVD 'Mythbuntu 14.04.2 LTS _Trusty Tahr_ - Release amd64 (20150218.1)' is required-
Please insert the above CD/DVD into the drive '/media/cdrom/' to install software packages from it."

The file is actually named "Mythbuntu 14.04.2 LTS amd64" (downloaded today).

Meanwhile, another dialog "Applying Changes" displays a half-finished progress bar with either the word "Donwloading" or "Finished" beneath it. If I insert the installation DVD And select "Continue" in the first dialog, the dialog flashes but is not dismissed. There is no problem with the internet connection, and I don't see why the installation disk is required (I have another Mythbuntu computer that added the Ubuntu Desktop without any problems and without requesting an installation disk).

I should note that I added the Mythbuntu and Myth repositories to the updater during installation and downloaded the latest updates.

This machine is used as a frontend and secondary backend. Installation was simple, and I am able to access the primary backend without any problems. I really like this product!