Hi. I have seen this error in sweeps before. You get this error when the mesh changes from one sweep to the other. Since your Ex is a function of x and y, to save all of the Ex on the same matrix, the size of x and y need to be same for all the sweep points. Since you are changing the radius of a circle, the mesh may vary depending on the radius. If that happens then the Ex for different radius will be defined on different meshes and the solver will not be able to load them in the same matrix. When you get this error, try to open the individual files and check if the mesh looks different for any one or multiple cases. If this the source of error then you can use a mesh override object to lock the mesh of the simulation region so that when the radius changes, the mesh will remain unchanged.