Hello,
unfortunately the newest iteration of HighSierra broke our fix of this issue yet again.
If it is happening to you, 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.
This should help until we manage to get a fix out.
Kind regards,
Ondrej Kolacek