I would assume that if the user closes their calendar in Outlook or closes Outlook and then tries again it will work?
Here is how Calendar flow works:
1. User creates appointment in Outlook
2. Exchange notifies BES
3. BES accesses the mailbox using MAPI and CDO
4. Appointment is sent to the BB
It's the same from BB to Outlook pretty much:
1. User creates appointment on the BB
2. BES accesses the users mailbox using MAPI and CDO
3. Appointment is created in the users Calendar in Outlook
If the user only sees this message in Outlook it is because he is looking at it in Outlook and when it reaches the users BB it is then "updated" with a Ref ID or if accpeted by a user then the attendee list is updated. The current appointment being viewed in Outlook is updated but will not allow for accepting/declining/updating until it has been closed and reopened.
You can get around this by not using the Preview Pane in Outlook. Or just let the user know when it happens to close the appointment and try again.
If this is not the issue you are experiencing then explain with more detail.