Convergence problem
 

I have a quite simple task in Flent: there is a cube, what is heated inside a box (aluminimum). The box is at open air. I tried to use Boussinesq-model to model air flow inside, and outside the box and it doesn't work, the calculation diverges after some steps. If I put a cube with similar geometry at open air the simulation runs fine, and it's converging. What can be the problem? The geometry is very-very simple and the mesh is excellent, that way I make something wrong in Fluent, I think. Can anybody help me?

Has anybody found a similar problem by Fluent?