If I create a JNLP slave and try to launch it using javaws from Java 7u51 on Linux, I get an error
Your security settings have blocked a self-signed application from running
As noted here, the remoting.jar needs to specify Permissions: full-permissions (I guess?) in its manifest, and needs to be signed with a trusted certificate (which apparently jenkins is not).
Workaround: run jcontrol, and under Security click Edit Site List..., and add the Jenkins root URL. Then try javaws again, this time dismissing the warnings.