OK, so the leak is coming from where the powerhead/motor part fits onto the canister, right?
So, you've replaced the rubber gasket that fits around the inside rim of the powerhead part? Make sure it's moist and not split. Make sure it's totally in that little groove - I had a leak on my 405 from the same problem a little while back.
When you put the powerhead/motor back on - push down hard, it
should be a tight fit. Make sure there is nothing obstructing the fit - for example, make sure the filter baskets are not overfilled and that they stack on top of each other perfectly. Make sure the lid to the filter baskets fits on evenly too - so that the round raised rim fits the impeller shaft tightly.
Hope this helps, let me know how you get on