Make sure the RPC daemon is running. On Unix, you can check this with:
$ ps -elf | grep unirpcd
You should see 'unirpcd' running. If you don't, find the 'startunirpcd' script (usually in /path/to/ud51/bin) and run it.
On *nix systems there has to be an entry in the /etc/services file that looks like this:
uvrpc 31438/tcp # UniVerse RPC Daemon
where the comment is optional.