I have just tried and found that
- Yandex uses an incorrect format for birthdays without year (
BDAY:--MMDD as vCard 3, which is invalid). So birthdays without year won’t be synchronized from Yandex to DAVx5.
- Some old contacts have a birthday in the Web interface details, but not in the Web interface overview. When syncing those contacts, Yandex does not transmit a birthday.
- When I set the birthday in the Web interface again and then sync, birthdays are working…
So it seems that Yandex’ birthday handling is not very consistent. If there are any problems, please provide exact steps to reproduce and ideally a test account.