I just ran into this during do-release-upgrade from 16.04.5 to 18.04.1 as well among other issues. The suggested mv solution worked.
I ended up in a situation where some to-be-upgraded packages like network-manager-gnome were removed at first but never reinstalled, which turned out to be problematic, although I'm not certain it was this interruption's fault that apt somehow "forgot".
I just ran into this during do-release-upgrade from 16.04.5 to 18.04.1 as well among other issues. The suggested mv solution worked.
I ended up in a situation where some to-be-upgraded packages like network- manager- gnome were removed at first but never reinstalled, which turned out to be problematic, although I'm not certain it was this interruption's fault that apt somehow "forgot".