I encountered this error when I tried to connect to CRM via the Developer Toolkit and Plugin Registration tool.
Upon closer inspection, the error was the tools weren’t able to resolve DNS for dynamicscrmoce.accesscontrol.windows.net.
For my particular issue, Windows Firewall was blocking the access because it recognized my home Wireless network as Public. The solution was easy, turn the Wifi connection to private network and access is allowed.
For list of Internet accessible URLs required for Dynamics CRM Online, please refer to the following link https://support.microsoft.com/en-us/kb/2655102.