Error: Could not synchronize record; Outlook 2011 on MAC

Hello
I use Outlook 2011 for MAC user that is showing the following error:
Error: Could not synchronize record: Discuss layout for new sampler packs to Exchange server
Details: The meeting request has already been sent and might not be updated.
Error Code: -19718
We are using Exchange 2010 Service Pack 1. Also these errors showing the user's Outlook is running as it is supposed to be. There were some extra errors pertaining to gathering requests and he found the requests and removed them. The errors recorded in the attached file are for gathering requests.
I searched in Google for the Error code and the information but I could not find anything on it.
Need a solution. Thanks in advance.
