I just signed up to the forum because we're having the same issue. Please fix ASAP as our clients can't work.
@AshC connecting to a webinar is not the problem (which is why https://support.goto.com/webinar/help/join-a-test-session-g2w060001 works fine) the spinning circle problem is only happening on the Registration/invite page.
I'm sorry we have no remaining support for Internet Explorer web browsers. There may be odd behavior if IE is used for scheduling, registering, or joining webinars that we cannot predict due to lack of testing and support from Microsoft. The browser MS continues to update will be Edge for Windows (this is based upon a Chrome framework and is free to download).