To resolve this issue, please follow these steps:
- Log out of your account out by selecting the icon at the bottom left side of the SketchUp drawing window.
- Log back in to your account.
- Try opening 3D Warehouse or Extension Warehouse again.
If that doesn't seem to do the trick, there may be an issue with the Internet browser on your computer. In this case, delete your browser's history: Internet Explorer for Windows users and Safari for Mac users.