Online meetings are created against a personal account for the meeting service. As a result, the Online Meeting Credentials documents are restricted to access by the author and the owner of the mail database. For mail databases delegated to a PA where a PA also creates online meetings on behalf of the owner, the expectation is that they will also set up the Online Meeting Credentials document or have access to the authentication details. If this is not the case, your Domino Developer will need to make a change to grant PAs access to the Online Meeting Credentials documents.
To confirm the owner of a mail database, open the preferences from the Inbox (More > Preferences…). The owner is named on the Mail > Basics tab.