Depth Test not working properly on Nvidia

Depth Test not working properly on Nvidia

So, I send a Development Test of my in-dev game to some friends, and they found out that the Depth Test in OpenGL does not work on Nvidia.
I use my own matrices and sent them to the shader, and at the start of evey game loop I use
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT ); // clear the display
You can see mountains through other mountains and stuff. On my Radeon HD 6650M it works perfectly fine. Any ideas?
I don't have anything special in the shaders, just some basic lighting calculations. I dont touch the gl_FragDepth.
Screen: (Yes, the textures are just placeholders :))

Here is my calculation for the Projection Matrix I use:
public Matrix4f getProjectionMatrix() {
    // Setup projection matrix
    Matrix4f projectionMatrix = new Matrix4f();
    float fieldOfView = 40.0f;
    float aspectRatio = (float)Display.getWidth() / (float)Display.getHeight();
    float near_plane = 0.1f;
    float far_plane = 1000f;

    float y_scale = coTangent((float) Math.toRadians(fieldOfView / 2f));
    float x_scale = y_scale / aspectRatio;
    float frustum_length = far_plane - near_plane;

    projectionMatrix.m00 = x_scale;
    projectionMatrix.m11 = y_scale;
    projectionMatrix.m22 = -((far_plane + near_plane) / frustum_length);
    projectionMatrix.m23 = -1;
    projectionMatrix.m32 = -((2 * near_plane * far_plane) / frustum_length);
    projectionMatrix.m33 = 0;
    return projectionMatrix;
}