You need to remove it again, or at the next change, and see that all four surfaces are flat and smooth. You can run a flat file carefully over both sides of the fitting and maybe on the transmission. The bolt will be harder to repair if it is not flat and smooth, but if the bolt looks bad, you can get one for not much at the MB dealer. In fact, it might be worth having a new bolt ready the next time you do the change. Dress the other three surfaces, use some new seal rings and a new bolt, and I will bet that it will no longer leak.