Altium 17 hatched polygon problem
Ok I solved it. Actually the problem was in the clearance rules. In the project there are several clearance rules for different components and basically the problem was the following: the clearance between pad and traces or pad and copper of some components were defined several time (shame on me) and the priorities were set in such a way to have for example a 0.3mm clearance for the rules with the highest and lowest priority, and 0.2mm for the rules with the priority in the middle, so apparently Altium didn't know exactly how to behave. So tidying up the rules solved the problem. Thank you to everyone!