Thanks for posting to the Telerik developer forums.
We are not enforcing any such requirements for the websites in question. My suggestion would be to examine the behavior (may be connected to old/changed DNS records for the domains which for some reason are not changed in your network or are outdated).
If it persists the issue may be rooted in the local network configuration or HTTP debugging (proxy) software installed. Please examine these settings on your side.
Also, you can try with another network/device or with subdomains like https://platform.telerik.com, http://www.telerik.com/documentation. This may shed more light on what exactly the issue is.
I hope this helps.
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