When creating an event the WebEx connector may throw the message:
Error registering with webinar provider. exception="WebEx API call error; Self registration is not supported by this service type or current meeting. [WebEx exception ID = '110051'] "
Please make sure the event in WebEx is set up to receive registrations. To do this you need to go to the event in WebEx and check the registration required checkbox.
Create an Event with WebEx - Marketo Docs - Product Docs