We've got the answer just for you. We had the same error (for daily format
only) and a collegue found the answer (quote):
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trying to print a collegue's Outlook calender in Day View Only. I receive an
error message. "The messaging interface has returned an unknown error. If the
problem persists, restart Outlook"

here's the fix:

In the colleague's Outlook calendar (Daily View)

Select FilePrintPage Setup button

In the Format tab, under the Options deselect the checkmark on the Taskpad.
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ToÃ*n Pham
University of Maryland University College
www.umuc.edu


"Dave S" wrote:

Hi, I have read-only access to my boss' calendar. I can read the calendar
fine, but get this error when trying to print:

"The messaging interface has returned an unknown error. If the problem
persists, restart Outlook".

This has been like this all week and Outlook was restarted multiple times as
was the workstation.

Thanks
Dave