Zwift runs “on top” of your default browser. So when Zwift teleports you into a riding world and it gets hung up at the “Loading (world) …” screen, then try clearing the cache of your default browser. (In my case that was Firefox.)
But just before clearing my cache, I switched my default browser to Edge (which I almost never use on that PC). Zwift returned to normal!
And then I went back to Firefox. Cleared its cache. Reset it as my default browser. Then went and restarted Zwift. Everything was working again in Zwift.