When you integrate GotoWebinar during setup, you are asked to enter your GotoWebinar credentials.
Please make sure you have entered your GotoWebinar details and not your Aevent login details.
Sometimes the system will autosuggest Aevent for you but you MUST enter GotoWebinar's credentials for the system to connect.
Another possible option is that you didn't allow the security to access as described here. It's imperative you do so!