I'm trying to add a windows slave to my master. when doing so, any of the start up commands fail:

1) Launch agent from browser on slave
2) java -jar slave.jar -jnlpUrl https://<hostname.domain>:443/computer/<windows_slave>/slave-agent.jnlp -secret <huge_hex_string>

In the 1st case it tells: "Unable to launch the application"
java.lang.NullPointerException
at com.sun.deploy.security.DeployManifestChecker.verify(Unknown Source)
[..........]

In the second case it tells:
Failing to obtain https://<hostname.domain>:443/computer/<windows_slave>/slave-agent.jnlp?encrypt=true
java.io.IOException
https://<hostname.domain>:443/computer/<windows_slave>/slave-agent.jnlp?encrypt=true doesn't look like a JNLP file; content type was text/html charset=utf-8

Any hint?
Thanks!