Perhaps the initial conditions that you have specified (flow and boundary conditions) result in zero depth somewhere in your reach. The RAS unsteady flow solution will not work with zero depths at any computational cross sections. Suggest that you run your model with steady flow (using the initial flow in your unsteady flow data) to check the depths. Another possibility is the occurrence of supercritical flow; that may cause the computations to abort. So, check the channel slope and initial flows for this condition.