When walking through the sewers with Triss in Novigrad, there is a room with lots of rats and wooden stairs. There are two gates leading further, however, both are locked.
I don't remember having encountered this problem when I had played the game earlier.

Any ideas??

You have to find another way through. Hint: Aard.

Which way? I know that you have to crush some walls on the way to the rat-room, but in this room there are two locked doors. No walls to crush. The rats kill me too fast.

Triss has to be there to open the appropriate door/s. Sometimes Triss is bugged and doesn't follow you. Make sure she is there as well. See here.

I found the answer: I did not know the rats can be killed