In this paper we consider optimal congestion control and routing schemes for multipath networks with non-congestion related packet losses which can be caused by, for example, errors on links on the routes, and develop a relaxed multipath network utility maximization problem. In order to obtain the optimum, we present a primal algorithm which is shown to be globally stable in the absence of round-trip delays. When round-trip delays are considered, decentralized sufficient conditions for local stability of the algorithm are proposed, in both continuous-time and discrete-time forms. Finally, a window-flow control mechanism is presented which can approximate the optimum of the multipath network utility maximization model.