Everyone that is connected to this exchange environment is receiving an error when they attempting to share a calendar in Outlook 2010.  We are using Exchange 2010 and Outlook 2010.  The error reads:  Error while preparing to send sharing message.  

 It works perfectly in the OWA when trying to share a calendar, 

Any ideas on a reason why this would be happening?

Hi,

I have noticed the calendar worked fine on OWA.

Please try to run outlook online mode to try.

Also we can recreate a new outlook profile to test.

I wonder to know if you can share successfully other calendar .

I suggest you right click the problematic Calendar > Properties.. > Permissions and add the user Reviewer permissions there first. Then please try sharing the calendar again.

Regards,

David