The incident was caused by Microsoft changed the Outlook Protocol from (RPC/HTTP) to (MAPI/HTTP) which provides better security and support for enhanced authentication on Office 365.

Issue description:

• Cannot connect to Exchange Online (Office 365) in Outlook 2013;
• Cannot access shared calendars that shared by the other users

Resolution:

• There are one important update that need to be installed for Outlook 2013, the update can be downloaded in following website.
  "KB2910923 - http://support.microsoft.com/kb/2910923". This update can fix many issue including cannot open shared calendars that shared by the other users when you configure Outlook 2013 to connect to your mailbox by using the MAPI over HTTP protocol. Besides that, there are many other issues can be fixed by the update as well, detailed information can be found in above website.
• Additional update:
  Please also make sure the "December 9, 2014 update for Office 2013 (KB2920734) update" has been installed on your system, if not, please download and install the update "KB2920734 - http://support.microsoft.com/kb/2920734" as well.