Solution for the error "The form required to view this message cannot be displayed. Contact your administrator" in outlook calendar 2007, 2010
Clearing the forms cache
The first thing you should try is clearing the forms cache. This will make sure that, in the case of using a custom form, the latest published version of your custom forms gets loaded again.
How to clear the cache ;
* Outlook 2007 and previous
Tools-> Options…-> tab Other-> button Advanced Options…-> button Custom Forms…-> button Manage Forms…-> button Clear Cache
* Outlook 2010
File-> section Options-> section Advanced-> group Developers-> button Custom Forms…-> button Manage Forms…-> button Clear Cache
When you want to clear it manually, delete all contents from the following directory when Outlook is close;
* Windows XP
C:\Documents and Settings\%username%\Local Settings\Application Data\Microsoft\FORMS
* Windows Vista and Windows 7
And then restart Outlook.
If you still have this issue after clearing the forms cache, make sure that Outlook is configured to open the correct form when creating a new item in that folder. To see which form is the default, right click on each of the Team Helpdesk folder and choose Properties. Here you’ll find a drop down list called “When posting to this folder, use:”. Making sure the following are matched;