O365 Outlook share error

We are on Office 365 and our users have Office 2016 installed.  One of our users is having an issue when trying to share his calendar.  When he clicks on share, the email pops up but when he hits send it triggers an "Error while preparing to send sharing message".

I saw an earlier solution about changing the mode from Cached to Online but I'm hesitant to do that.  I don't want to slow down the email client.  The user does have two email accounts and both are set to Cache.  

Thanks for any help.
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Vasil Michev (MVP)Commented:
Simply try to share via the Outlook Web App, does it work OK there?

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