As a delegate, you can't open a meeting request in Outlook Web Access (OWA) that was forwarded by your manager who is in a different forest. You can't accept or decline the meeting request, and you receive the following message in the preview pane:
Your request can’t be completed right now. Please try again later.
This issue occurs because delegates and managers may not be moved to the cloud at the same time. OWA doesn’t support the ability to accept and decline meeting requests.
To resolve this issue, use the Outlook for Windows client to accept or decline the meeting request.