If you work with Outlook 2007 in combination with Exchange, you might get the following error when trying to open a shared calendar:
“The messaging interface has returned an unknown error. If the problem persists, restart outlook“
To open the shared calendar anyway, you have to close and re-open Outlook. It will now work again (for a short while).
Update march 6, 2008: There’s a hotfix available for this messaging interface problem! It’s not a public hotfix yet, but you can request the hotfix. The hotfix includes a solution for the message you get when you try to open a shared calendar in Outlook 2007:
The messaging interface has returned an unknown error. If the problem persists, restart outlook
For more information and to obtain the hotfix, go to