Dave,
I've had this happen a couple of times when running shared buffers. Drop to a DOS prompt (assuming it's a winblows system you're on), switch into your dnetc directory and type "dnetc -forceunlock buff-out.rc5". The same applies for the in buffer as well if you have a problem there.
A couple of things can cause this to happen but I've found shared buffers between multiple clients to be the most likely, at least in my experience anyway.