It happens immediately after I click on the big green "Enter Room" button and no I have not tried using the temporary client instead of the real client. If the only way to pick the temp client is to remove the real client, then I'll have to try that ...
I'm having the exact same issue, and all I could find on the internet was this post, and only one other on a Microsoft site: https://social.technet.microsoft.com/Forums/en-US/6a40f5e6-d440-488e-a23c-d6c956e66733/webex-in-windows-10-not-working-anymor...