Wrong camera rotation in flythrough mode (scene view navigation).

Hi,

I'm currently working on a 2D scene in Unity 2017.2.08b (2017.1 did not start on my system).
Thus the terrain is using the X/Y plane, to see it from the top when going into 2D mode of the scene view.

However sometimes I want to look from a 3D perspective at my terrain (just in the editor), thus I leave the 2D mode and use the flythrough mode (holding right mouse key) and WASD to navigate in my scene.
Unfortunately the scene camera rotation is somehow strange: When I move the mouse along the x axis, the camera does not rotate on the Z axis, but on the Y axis. Thus I, when my terrain (X/Y plane) is below the camera, I cannot "look" right or left, because looking right or left just rotates the camera.

Is there any option to rotate the camera around the z axis instead the y axis, when being in the flythrough mode of the scene view navigation?