Exception when adding a Message Object to an Attachments collection

Hello,

We have encountered an Exception while trying to add a Message3 object
as an attachment to the attachment collection of the message draft:

"Dieses Verfahren bzw. diese Eigenschaft wird nicht unterstützt." which
probably roughly translates to "This operation is not supported"
at System.RuntimeType.ForwardCallToInvokeMember(Strin g memberName,
BindingFlags flags, Object target, Int32[] aWrapperTypes, MessageData&
msgData)
at GroupwareTypeLibrary.DIGWAttachments.Add(Object vr, Object kind,
Object DisplayName)

We are connecting to a Groupwise 8 Client via the Groupwise Object API.
The method call looks like this:

draftmsgAttachments.Add((Message3)attach.messageRe ference, null, null);

draftmsgAttachments is an Attachments object, attach.messageReference is
a Message3 object.

This method worked while connecting with a Groupwise 7 interface, we
believe. For some Reason I also can't call the Attachment Add(Message
Message, [String DisplayName]) because neither the Attachments nor the
Attachments2 interface show this method signature, although for examlpe
the Recipient interface shows the RecipientStatusList, which means the
Library is on Groupwise 8 level.

Why does this exception happen?

thanks for the help.

Best regards, Martin Schmidt.