Z-Axis blocked on MMB movement While panning the orbit with the MMB, the movement of the start-up file cube, as well as of any other object, is restricted only to the X and Y axes, while the Z axis is blocked. What have I done wrong? I could always move my objects with the MMB along all axes. Trying to reset, I have downloaded the latest version v2.79b but unfortunately the problem persists. I've searched for an answer, but found only ones that relate to extrusion
Thank you for your help.
You seem to be referencing the cameras movement in the viewport accessible via  MMB, as said in your comment.


  Mr Zak, the Z axis is blocked only when rotating the viewport.


By default, the Viewport Orbit Style is set to Turntable. For  a less restrictive Orbit Style, set it to Trackball.


  [The Orbit style sets] how Blender works when you rotate the 3D View by default when holding  MMB.
  
  Turntable
  Rotates the view keeping the horizon horizontal.  
  
  This behaves like a potter’s wheel or record player where you have two axes of rotation available, and the world seems to have a better definition of what is “Up” and “Down” in it.
  
  The drawback to using the Turntable style is that you lose some flexibility when working with your objects. However, you gain the sense of “Up” and “Down” which can help if you are feeling disoriented.
  
  Trackball
  Is less restrictive, allowing any orientation.


Blender Manual

To set the Orbit style to Trackball, open the User Preferences window &#9096; Ctrl&#9095; AltU and go to the Input rider. Then click on the Trackball button.