When trying to use apt-ger earlier, I receivd the following:

dpkg: unrecoverable fatal error, aborting:
syntax error: unknown group 'Debian-exim' in statoverride file
E: Sub-process /usr/bin/dpkg returned an error code (2)

As it turns out, the passwd and group files had been copied from another machine, without properly adjusting the apps already installed on this one. Since statoverride had some options configured in it, this left apt in a poor state. The simple fix was just to remove the Debian-exim group from the statoverride file:


Once removed, I could successfully use apt-get again.