Scheduling and managing events using Outlook.com calendar
Dear Stefan,
the sync problem between People Hub and Birthday Calendar has been there for years now.
Deleting and recreating the entire Calendar never made any difference for me. If you wait a few hours, the Calendar should repopulate - but don't expect it to be complete, birthdays events will go missing now and then.
What would really make it worse is deleting a specific birthday from the Calendar: in that case you will never be able to add a birthday for that specific contact. You will have to create a new contact (with the same name, if you want) and add the birthday again to the newly created contact. On this very thread, a Microsoft employee confirmed that this is by design, to my disbelief.
In short, to my limited knowledge, there is not a known solution to the Outlook.com Birthday Calendar sync issue as of today, - literally years after the problem was first reported.
Best,
Fra