I found the workaround from this thread to be successful: Virtual Webcam no Longer Works.
Basically, if you unsign the latest Webex desktop client, things work.
Command would be: sudo codesign --remove-signature “/Users//Library/Application Support/WebEx Folder/T33_64UMC_40.2.16.14/Meeting Center.app”
Where you replace the with the name of your user folder.