If you know the IP Address and Port Number in the QLAClient.properties file are correct, and you know that the QLA Server is running correctly, relaunch QuarkXPress to see if it will connect.
There are several different reasons QuarkXPress may not be able to connect to the QLA Server. When QuarkXPress connects to the QLA Server, it creates a Session ID. If for some reason (network data collisions or signal attenuation, incorrect IP address or Port Number), the session breaks or expires and is not refreshed, QuarkXPress tries to connect with the expired Session ID. This can cause this error, which indicates that the client is trying to connect to the QLA Server with an invalid session key.