I can't tell what is causing the error for sure. Commonly, such errors are caused by a proxy settings, cached files from old installation of the demos or by firewall settings. This is why I would suggest you to try the following and let me know if the issue still persists.
- Stop your firewall and try to install the demos again.
- Change your proxy settings to allow downloading from the demos server.
- If you have an older version of the demos, uninstall it. Then install the new version.
Also, you can try to install the demos on another machine without any specific network set up and see if the issue reproduces.
Telerik by Progress
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