Comment 12 for bug 1719863

I had the same problem on 18.04. I noticed that the version it tries to install is 3.6 already, so it isn't the wrong version what's the problem here. Than I realized that apt uses ipv6 addresses...
Forcing apt to use ipv4 solved the problem for me. (https://unix.stackexchange.com/questions/9940/convince-apt-get-not-to-use-ipv6-method)
What I've done exactly:
$>sudo rm -rf /var/lib/update-notifier/package-data-downloads/partial/*
$>sudo apt purge ttf-mscorefonts-installer
$>sudo apt-get install -o Acquire::ForceIPv4=true ttf-mscorefonts-installer