BoxMesh solution differ from gmsh Mesh solution

Dear all,

the example domain with different materials shows a solution as expected if I use a BoxMesh mesh.
But the solution with a gmsh mesh differ regardless of using a coarse mesh or fine mesh.

The values of the solution variable are almost the same but the gmsh solution follows the mesh.
Furthermore the "flux" of the solution variable shows outwards arrows on faces which are in flux directions and incorrect values at corners.

What do I wrong, or how can I improve the solution?

For a further simulation I need an imported mesh. But first I have to handle this "simple" example.

Best regards

Michael