I'm currently trying to subscribe to my Google Contact's Birthday calendar from Hotmail, so far without success.
I've used the ical link from the calendar settings in Google to create the subscription and it appears to create successfully.
Going back into the calendar settings in Hotmail the same error is repeated five times:-
Errors: This calendar wasn't updated because of a problem with the
publisher's file. We'll try updating it again later.
A search on that phrase hasn't turned up anything useful to date.
Best Answer
Currently this isn't possible due to the fact that the iCal address for your Contact Birthdays Calendar isn't a personalized URL.
This is the url that you get for Google Contact Birthdays:
https://www.google.com/calendar/ical/%23contacts%40group.v.calendar.google.com/public/basic.ics
This will only work from within a Google calendar page.