Hello @badger ,
Is it possible there is an ad blocker/VPN/Firewall that is preventing the connection. How are you attempting to connect. Using Neo4j Browser in a Web browser? Or using the embedded sandbox in the GraphAcademy course? By now, your sandbox for a course would have expired (3 days), but if you refresh the GraphAcademy course page, it will recreate and re-initialize the sandbox.