Found this when I tried to sync google public calendars with
syncevolution (as an ics subscription replacement).
It seems that google still requires authentication even for accessing
public calendars. ie:
https://www.google.com/calendar/dav/vi6voh10oa187gg9nvs8pkqt7s@group.cale...
Even with one-way-from-remote it exits with 403 (Auth required). The
synchronization is successful.
If required, I could attach the log.