Something is wrong with your DNS, -
As per default, your network (e.g. your router) will be using your ISP's DNS servers to resolve names for you, where local machines will be using then be using your router's internal "forwarder".
Sometimes those DNS servers you use may be overloaded, or they may temporary be out of service (e.g. due to maintenance, where your ISP is working on one of the typical 2-5 servers they provide).
You can try to change the DNS settings on the system running your game server, to use something else, e.g. like the public DNS servers provided either by
OpenDNS or
Google Public DNS.
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