This error is probably caused by no network. Most of the services, especially those 3D tiles cache services, are online services. Therefore, if your PC cannot connect to internet, you would encounter this error.
You can simply check 'config.js' (examples\js\config.js) to find the required url.
By the way, if you won't connect to internet, you can download an iServer on official website. And the following steps are like these:
1. unzip the iServer
2. copy webgl under iServer\webapps
3. run iServer\bin\startup.bat to launch the server
4. visit the sample url like http://localhost:8090/WebGL_EN/examples/examples.html
then you can check the samples and enjoy webgl 3D.