The process of labeling appointments (Birthday or None) takes not place directly in Outlook® so one could think that nothing changed after the adaption.

The birthday list is created with the birthday entries in the calendar not with the birthday entries in the contacts. So deviations are possible.

If you get that message, there's probably an incorrect element in your calender or contact data.

It is a matter of not identified birthday dates. 

With deletion of birthday corpses only those entries are deleted that aren't any longer linked to a contact.