We are seeing the same issue with Windows and Chrome. When launching Webinars, as a Host, the Zoom client never opens. No error message. Nothing.
Seems to be related to this in Chrome: Launched external handler for 'zoommtg://vanityURL.zoom.us/
Chrome on Mac and Linux work as expected. Issue affects Chrome for Windows.
Our workaround has been to use Firefox where we’ve seen no issues starting Webinars.