. . . . a paper jam.
If this message has just started after a long period of ownership, I'd dismantle the whole thing including toner carts, fuser unit, transfer belts imaging drum, paper trays etc etc etc, give it a good brush and vaccuum out, then inspect EVERYWHERE for a thumbnail sized piece of paper which may be lodged somewhere fairly well hidden.
We've had a 4550DN in our office for several years, or should I say we've had several 4550DNs in our office over the years and they've not been very reliable, each successive 'broken down' model being replaced by a reconditioned unit.
I've had too many error messages to mention, including many from newly reconditioned units, so give it a good check over - if you can't find the problem, put it down to one of those annoying self diagnostic error messages that are designed to be more confusing than helpful.