I’m reposing this because I found it to be the easiest way to handle this kind of problem (in most cases):
It is essentially to follow exactly as I wrote, nothing more nothing less.
- Close your SketchUp, then reopen SketchUp. Wait until fully loaded.
- Select the menu with mouse: Window>>3D Warehouse
- Hold the ⊞ Winkey and hit the →right arrow key several times until the 3D Warehouse windows appears.
The most important is to do not click with your mouse or do not hit other keys as written.
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In the other cases when the above wont work, (e.g. plugin dialog or Model Info, Preferences missing) this is more complicated, but more “sure”:
https://forums.sketchup.com/t/extension-manager-warehouse-wont-open/278207/2?u=dezmo