I'm by no means an espresso expert ( I currently don't actually own an espresso machine ... )
But I believe pressurised baskets are meant to account for coffee that isn't ground fine enough, the added pressure is making up for a puck that doesn't provide enough resistance to the water. It may be that you're able to grind fine enough that the added pressure from the basket is causing water / coffee to escape out the sides.
Maybe try grinding coarser ( like pre-ground / pour over coarse )?