It's possible that it is the same issue as http://webwork.maa.org/moodle/mod/forum/discuss.php?d=4501, but I don't think so because you are getting a different error message.
First thing to check is if the R server is running. If it is, then the next thing to check is security settings: Is there a firewall blocking connections (unlikely if Rserve is running on the same server as WeBWorK)? Is there some security policy (e.g. SELinux) preventing connections?