Try again, sometimes network connections are flaky or servers are not available on occasion..
It is probably your proxy configuration /setup for the internet connection, however as you say apt works, why not just install directly from the terminal
apt-cache search mscorefonts
msttcorefonts - transitional dummy package
ttf-mscorefonts-installer - Installer for Microsoft TrueType core fonts
--or maybe you were using the incorrect file name