It's a bug they are working on. It's caused by an error synching email addresses in the blocked sender and safe sender list between Outlook and Hotmail. It does not stop synchronization of the mail, calendar, contacts and other data (including the junk lists). In the majority of cases the issue is related to email addresses that have an underscore in the domain name – the server refuses them. In other cases, there is a malformed address on the list (ie, missing .com). So... check your blocked and safe lists for addresses with underscores or that are incomplete.