Update:
I have another system running macOS 10.14.4 with Virtualbox 6.0.0. I performed a fresh installation of Linux Mint 18.3 Mate, applied all the OS software updates, installed 6.0.0 guest extensions, and then installed calibre 3.40.1 using the command:
sudo -v && wget -nv -O-
https://download.calibre-ebook.com/linux-installer.sh | sudo sh /dev/stdin
calibre installed without any apparent issues.
Presumably something happened between VB 6.0.0 and 6.0.4 to break a calibre 3.40.1 installation.