Ubuntu :: Error Icon In Gnome Panel From Apt Not Go Away

I have an error icon in my Gnome panel that will not go away. It appeared after running an apt-get command (explained below). Everything is working fine, but I cannot get the error icon to go away. Surely there must be some file somewhere storing the state that is making this reappear. The icon occurred after the following: I recently upgraded to Ubuntu 10.04. Yesterday, I decided to add the Medibuntu repository to my apt sources based on the instructions at [URL] -- i.e., I ran the command:
sudo wget --output-document=/etc/apt/sources.list.d/medibuntu.list [URL] && sudo apt-get --quiet update && sudo apt-get --yes --quiet --allow-unauthenticated install medibuntu-keyring && sudo apt-get --quiet update

In the terminal window, I saw the following error message amidst the other output:
W: GPG error: [URL] lucid Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 2EBC26B60C5A2783
However, everything appears to have worked fine. The Medibuntu repository has been added to my sources, the keyring has been added, and I am able to install packages from the repository using apt-get without any errors.

Nonetheless, an error icon appeared in the Gnome panel after running the above command-line, and it will not go away. If I move the mouse pointer over it, the following text displays:
An error occurred, please run Package Manager from the right-click menu or apt-get in a terminal to see what is wrong. The error message was: 'Unknown Error: '<type 'exceptions.SystemError'>' (E:Opening configuration file /etc/apt/apt.conf.d/99synaptic - ifstream::ifstream (13:Permission denied))' This usually mean that your installed packages have unmet dependencies

The file /etc/apt/apt.conf.d/99synaptic it mentions is present and is readable (at least as root). Running Package Manager works fine with no errors. I've tried running
sudo apt-get check
and it runs fine without any errors. Likewise,
sudo apt-get update
and
sudo apt-get upgrade
run fine with no errors. But the error icon remains.
If I log out of Gnome and log back in, the error icon returns. If I reboot, the error icon returns as soon as I log back into a Gnome session. How to I get rid of this?