3 things!
First, be CERTAIN you did not overload the baskets, better to be 1/4" low than 1/32" high.
Second, the connector goes attached to the base of the head. The rubber gasket may tend to stick to the connector at the head, so everytime you open it you need to pull it out and put the rubber part back onto the basket because the rubber gaskets are likely to warp and seat badly if you leave it on the connector at the head.
Third, be sure the two baskets are in the right spot, I found that one basket is ever so slightly smaller than the other, that is the lower basket, it is easy to mix that up when you first load them and then the bottom basket does not fit quite right. So, test, remove both baskets, then put one in then try the other to see which fits lower into the canister, that fraction of an inch makes a difference.
Also, do not use Vaseline from the drugstore, but the Eheim stuff is fine. I buy slicone lube from the pet stores for o-rings and such. Yes, it will harden the rubber.
Sad to admit that I've done ALL of these things....
and I cracked the impellor cover also, when I overfilled the basket, and it warped the basket but that relaxed eventually.
Never had a leak from the input connector, but I am suspecting overfilling might cause that. Guessing, though.