Outlook 2013 does note sync contact birthday to outlook.com for contacts copied from contact folder

I wanted to switch my contact management from OpenXChange/Outlook to Outlook.com/Outlook.  I'm using Outlook 2013 (15.0.4615.1000) on Windows 8.


I followed the steps described here: http://support.microsoft.com/kb/2844991/en-us and copied all contacts from local Outlook folder to the contact folder of my outlook.com account.


I turned out, that the contacts were synced, but the birthday date for all of them was gone. Even manually entering the date in Outlook did not fix it.


How-I-fixed-it:
- Export all contacts to a csv-file from Outlook
- Manually import the contacts from the csv-file in outlook.com