Currently the WebEx calendar connector for Exchange only supports only "Legacy" authentication (Basic/NTLM).
Microsoft recommends customers using on-premises MS Exchange servers are changed over to use "Hybrid Modern Authentication" and for "Legacy" authentication to be disabled. This enables customers to enforce features such as multi-factor authentication on their on-premises Microsoft Exchange.
OAuth (Modern) authentication already exists for the Exchange calendar connector server for Office 365.
Request is to add OAuth/Hybrid Modern Authentication support for the on-premises WebEx Exchange Calendar Connector.