I'm not sure what to suggest: if you give it the -s option, slimrio doesn't do anything fancy; it just sends UDP packets to port 3483 on the specified host, so it's hard to think of anything that could be wrong on the slimrio end. Is there anything in the server's log? Does the receiver show up on the server's list of recognised players (on the status page)?

Perhaps a tcpdump of traffic between the rio and squeezecenter would shed some light on what's going on.