Disconnecting com.mohang.authlib.GamProfile / Authentication Servers are Down

After launching a server with the latest 1.12 server.jar file, the ability to connect to the Authentication server will fail.

Server was launched from Putty using java -Xms1G -Xmx4G - jar minecraft.server.jar nogui

Error thrown up is:

User Authenticator #1/INFO: Disconnecting com.mojang.authlib.GamePr ofile@25c9af45[id=<null>,name=USER,properties={},legacy=false] (/ADDRESS): Authentication servers are down. Please try again later, sorry!
[07:07:55] User Authenticator #1/ERROR: Couldn't verify username because serve rs are unavailable

The server successfully launches, and players can see that it's operational, but are unable to connect: "Took too long to log in."