Scenario: Customer is running Exchange 2010 and Exchange 2013 servers in coexistence. The servers have the latest rollups and patches. Users using Outlook 2013 with mailbox on Exchange 2013 are not able to edit calendar of an Exchange 2010 mailbox user.
Note that customer is running Windows 7(sp1) and Outlook 2013 sp1
Resolution: Run windows update and ensure Outlook and Office 2013 are all patched to this version: 15.0.4615.1000