[dkarp]

Does it work to login as robert via the web UI and modify the appointment in that way?

Alternatively, auth as robert and do your command via SOAP, qualifying the appointment ID that you're acting on (i.e. instead of id="777", use id="{toyota-zimbra-id}:777").

Or auth as robert and go via SOAP, adding an <account by="name">toyota@abak.net</account> element to the <context> block in the SOAP header.