OWA users not able to share calendar

Hi,

We moved our Exchange 2003 environment to Exchange 2010 SP1.

Part of our users are using OWA as a mail client. Some of them are not able to share there calendar.

This is how far the get: rightclick calendar -> share -> share calendar.. -> type a username in to and when the click send, the message is:

A problem occurered while you were trying to use your mailbox.

And the message is not send.

When I look at the mailbox of this user using powershell, one thing is strange:

Get-MailboxFolderPermission -identity user1:\Calendar

results in:

Get-MailboxFolderPermission : The operation couldn't be performed because object 'user1:\Calendar' couldn't be found on 'Microsoft.Exchange.Data.Storage.MailboxSession
'.
At line:1 char:28
+ Get-MailboxFolderPermission <<<<  -identity user1:\Calendar
    + CategoryInfo          : NotSpecified: (:) [Get-MailboxFolderPermission], ManagementObjectNotFoundException
    + FullyQualifiedErrorId : 784DC3C3,Microsoft.Exchange.Management.StoreTasks.GetMailboxFolderPermission

--------

If use: Get-MailboxFolderStatistics -Identity user1

part of the output is:

Date                              : 09-Apr-11 15:56:20
Name                              : Calendar
FolderPath                        : /Calendar
FolderId                          : LgAAAADPw7GD3VqcQpV0xi55uk6wAQDaHPln6dWjRrEcPXAMcv/+AAABO7TtAAAC
FolderType                        : Calendar
ItemsInFolder                     : 787
DeletedItemsInFolder              : 0
FolderSize                        : 2.528 MB (2,650,280 bytes)
ItemsInFolderAndSubfolders        : 787
DeletedItemsInFolderAndSubfolders : 0
FolderAndSubfolderSize            : 2.528 MB (2,650,280 bytes)
OldestItemReceivedDate            :
NewestItemReceivedDate            :
OldestDeletedItemReceivedDate     :
NewestDeletedItemReceivedDate     :
OldestItemLastModifiedDate        :
NewestItemLastModifiedDate        :
OldestDeletedItemLastModifiedDate :
NewestDeletedItemLastModifiedDate :
ManagedFolder                     :
TopSubject                        :
TopSubjectSize                    : 0 B (0 bytes)
TopSubjectCount                   : 0
TopSubjectClass                   :
TopSubjectPath                    :
TopSubjectReceivedTime            :

TopSubjectFrom                    :
TopClientInfoForSubject           :
TopClientInfoCountForSubject      : 0
SearchFolders                     : {NoArchiveTagSearchFolder8534F96D-4183-41fb-8A05-9B7112AE2100, Restriction(1-632E21C), AllItems, Reminders...}
Identity                          : user1\Calendar
IsValid                           : True

So there is a folder Calendar, but Get-MailboxFolderPermission -identity user1:\Calendar, can not find it!?

results in calendar info.

How can I change this behaviour? Is it related to the share calendar issue?