Troubleshooting License Server Failure - "Cannot obtain ticket from ... due to connectivity problem: Server response is empty, incomplete or corrupted"

I'm trying to set up a license server in a high school computer lab. The server will run on the teacher's computer and for testing I'm trying to obtain a license for an instance of IntelliJ running on the same machine - which is running Windows 7.

The license seems to be granted on the server, but not received by the client.