When trying to expand a users Online Archive in Outlook 2013 you might receive the error message :
The set of folders cannot be opened. Microsoft Exchange is not available. Either there are network problems or the Exchange server is down for maintenance.
This occurs due to a bug with an update Microsoft deployed on the 12th August 2014. Deploy the update in the KB below and it will resolve the issue.
Microsoft Support KB : 2889859
Source : KB 2992644