This error can occur if the public foldes are not available, such as during maintenance. Verify if the public folders are online and functioning. This could potentially occur after a migration as well if the Schedule+ free/busy system folder was not properly replicated to the new server.
Speaking to Microsoft support about a different issue, they informed me that this message is what you will receive if you have two users trying to update something in the public folders at about the same time. The second person to try to save would get a conflict message and cause this event log.
Error: 0x80004005 = "Unspecified error" - The reaseon could be that the System Public Folders were deleted and could not be recreated.
Error: 0x80040115 = "MAPI_E_NETWORK_ERROR" - This may indicate a network connectivity error.