Ran across the following in the thread Using 3D warehouse products in 20-20 design software —
colin SketchUp Team
Some good news. The issue that had been introduced by changes to comply with more recent versions of Chromium, has been fixed. SketchUp 2017 and 2018 are showing the 3D Warehouse window contents ok now.
For 2020 Design in particular, I found that I needed to go to https://3dwarehouse.sketchup.com/ in the Chrome browser to do the sign in step, then the window in 2020 Design worked ok. You may need to even sign out, then sign in with Chrome.
I launched SU2017 and sure enough, the Warehouse opened up when I clikked the 3DWH button in the toolbar. And it seems that everything loads properly. However, I still cannot sign in - there is still the message “Couldn’t sign you in This browser or app may not be secure. Try using a different browser. If you’re already using a supported browser, you can refresh your screen and try again to sign in.” And of course, if you want to pull a model straight from the Warehouse into SU…you still have to be signed in, just as you do in a regular browser.