Hello,
unfortunately the new iteration of HighSierra broke our fix of this issue yet again.
Please run the terminal app and in it run the two following commands:
sudo sh -c "echo '*.icloud.com https' >> /Library/Application\ Support/Avast/config/com.avast.proxy.excl"
sudo killall -hup com.avast.proxy
Please note that it will prompt you for a password. After that, restart Safari and all should be fine.
We are really sorry for this, we will fix this ASAP, but this should help in the meantime.
Kind regards,
Ondrej Kolacek