Scenario: A mailbox on Exchange 2013 SP1 cannot open or edit calendar entries of an Exchange 2010 SP3 mailbox using OWA (Outlook Web App). Attempting to open the calendar entry does not appear to do anything. The user has explicit rights to this shared calendar.
Cause: Per Microsoft, this is an unsupported configuration for OWA. This is by Microsoft design.
1. Use an Outlook client to view calendar information between the 2013 and 2010 mailboxes.
2. Move the Exchange 2010 mailbox to Exchange 2013.