Hey NC, Ive heard that in the 6900 (previous model), tearing of the group head seal was often caused by a split in the group head itself. I havent heard of this happening to the 6910 and it would probably be obvious - but worth a quick check!
Other thing is that the seals do soften with age/use, so youll notice that the portafilter locks in further to the right with more use. I found that until it was locking in past the perpendicular, the seal did leak a little, just dripping a total of around 5ml mainly from the side opposite the p/f handle - so it may resolve itself with a bit of use. Other than that I can only guess that maybe the new seal isnt seated properly - have you tried removing and re-seating it?