@Liviu Lalescu I cannot succeed with my new schedule. Could you check what's wrong with it or try to launch a multiple session with your Pc?
I am sorry, I cannot do that.
The constraint "teachers - max gaps per week = 2" seems to be a problem. If you modify the constrains and set "teachers - max gaps per week = 5" the timetable solves quite easily (in less that 5 minutes).
Vangelis.