I have simplified the scenario on bellow picture:
[![enter image description here][1]][1]

I have line Bezier Curve which I'm using as a path for camera to moving along it. When camera is on top, it is pointing to down, and when it is coming down, I have turned it 90 degrees to see site from the box.

But when camera is on that position you see I'm unable to rotate it using x-axel.

I have two examples.

1. On the following picture I have used x-axel to rotate the camera landscape, but as you can see from the picture, it turns over the z-axel:

[![enter image description here][2]][2]

2. And, when I'm using z-axel to rotate, it does that as expected:

[![enter image description here][3]][3]

When the camera is on top, it rotates correctly. What I do not know on this?


  [1]: https://i.sstatic.net/EfY1w.jpg
  [2]: https://i.sstatic.net/MBOvv.jpg
  [3]: https://i.sstatic.net/AVKnY.jpg