Calendar Can't Save the attachment "....." to the Exchange Server

My boss just had the following error appear, and has asked me to figure out how to get rid of it. I have been looking for a couple hours across different forums, but there seems to be no concrete answer. I have not had a chance to try anything, as he has been busy, but I would be interested to know if anyone has encountered this error, and what they did to fix it.

Thanks in advance for any help!