Since upgrading to CU2 on Exchange 2013 several users have come back to me saying they get errors when trying to forward a calendar, here is the error:
This message could not be sent. Try sending the message again later, or contact your network administrator. Error is [0x80070005-00000000-00000000]
MS have advised this is a know bug and will be fixed in CU3. You can check if you get the same error on OWA as well. CU3 is scheduled to be released in about 3 months time.
I find that copying the actual content of the meeting request and then sending a new one works but for users that is a pain.
Hope it helps