This error usually occurs after a simulation runs for a long time, like several hours. StimPlan stops working at some point, but the simulator (the black prompt window) still runs.
This is a known issue, but unfortunately we have never been able to solve it. For some unknown reason, Windows just shuts StimPlan down. Often you can mitigate this problem by closing the two graph windows (Contour Plots and Nolte-Smith Plot) that are showing the simulated results during the simulation. Most of these problems occur due to communication clashes. We have setup the software to gracefully re-start. If the simulation is still running when you return and find the StimPlan has shut-down, just restart StimPlan and it will pick up and it will be as if the shut-down had never occurred. If, during the time while StimPlan was shut-down the simulation has completed, again just re-start StimPlan and it will pick up the final results and all will still be good. Thus, in either case simply re-start StimPlan and ignore the shut-down. We are hoping the completely new code will solve this problem later, but we have no idea what the problem is that triggers this at this moment.