Also could be a graphics driver issue. You could try updating your graphics drivers as well.

Are you running it through Citrix or any sort of streaming service? There was a common issue a while back.https://help.webex.com/en-us/article/WBX7874/Unable-to-See-Streaming-Content-Shared-through-Application,-Screen,-or-DesktopSharing

Hi, not sure if you figured it out but there are instructions here:https://help.webex.com/en-us/article/WBX9000035639/Unable-to-Schedule-Meetings-after-Upgrading-to-a-Paid-Plan