We found out something important. 3D Warehouse now seems to need at least version 88 of Chromium, which is built into SketchUp. The update to version 88 happened when SketchUp 2021.1 was released. If you are on 2021.0 or 2021.0.1, you would be seeing the problem.
There were two more updates to 2021 after 2021.1, if you’re going to update you may as well go to the latest version of 2021. Here are direct links to the Mac and Windows installers: