I am not sure what is causing your problem - I have never seen a report like this one before. The error implies that it wasn't the webex admin that created the meeting (all actions taken by the plugin are actually done by the admin account you set in Moodle settings page). So I would guess something is configured incorrectly, but I'm not sure if it is in Moodle or WebEx.
To be clear - we are a University that wrote this plugin for our own use, and publish it for the community to use. We don't have the resources to provide general support - I use my personal time to help when I can. This plugin is not affiliated with WebEx in any way.