I can´t access to this link anymore, Is it in another place now? Do you have the link where I can download the certificate?
Are you running a particular SOE for your workstations?
If so do you have access to a vanilla SOE eg on the domain and just office, Webex Tools, no other corporate/3rd Party applications and see if that works?
I have heard of this before also just chasing up what the fix was for their environment.
UPDATE: Customer was running desktop/device management application such as AppSense which was blocking the new plugin. If you have something this on your SOE try disabling that service on the local machine then attempt to join the meeting to identify.