Some basics on convex hull tests for folks that might read this later:
A convex hull is a volume defined by a number of planes (at least 4, but it could be much more). Testing to see which side of a plane a given point is on is trivial (computationally speaking), it's just a dot-product. Go look up "dot product" if you don't know what that is. Do it now. It's important.
Back? Okay, so in a working convex hull test, all the planes would be facing inward or all facing outward. In other words, for a point to be inside, all the dot products would be positive (which is what codymanix is using above), or all negative. You'll need to pick one in advance and stick to it, and all your data defining your hulls needs to stick with it too.
Codymanix is starting with a box, which is defined, as far as we're concerned, by six planes. If all six planes are "facing inward" then we're fine, but what happens when 2 or more of those planes are "facing outward"? All of a sudden, no point in our coordinate system will ever satisfy the positive dot product check on all six planes defining the hull.
To visualize this, lets imagine a square in 2d. The top is labeled "top" and the bottom is labeled "bottom". To be inside this square, a point must be below top and above bottom. With me so far? Okay, not lets move the top until it is below bottom... and we'll keep the tests the same. Now there is no point that is above the top and below the bottom. We've turned our square "inside out" and nothing can be inside it.
I suspect that condymanix's hull has been turned "inside out".
Okay, that's the problem, so how do we fix it?
IIRC, the "front" and "back" of a given triangle are defined by a "winding rule". "Wind" as in "wind your watch" not "wind is blowing". There are two different winding rules, called "left handed" and "right handed". Make a thumbs up gesture, fingers curled into a fist, thumb sticking straight up. The way your fingers curl is the order in which the points on a triangle are ordered to define the "front" of that triangle, the direction your thumb is pointing. Right hand: counter clockwise, left hand: clockwise.
Codymanix, I'm guessing you're mixing left and right handed winding. You need to define your hulls such that the thumbs are all pointed in the same direction. It would be nice if you use the same winding rule as your meshes just for consistency's sake. It will probably also make generating hulls from your meshes easier.