Talk to Janosh to receive a piece of paper with the code. Enter the ship and choose Porta Fisco on the radar. When you've made it to the area, talk with a thug standing next to the entrance to the sewers - he will tell you that there is a rebel hideout inside the sewers. Click on the cardboard boxes next to the entrance to the next area to get the ninth puzzle fragment.
Enter the sewers and take a look at the piece of paper which was given to you by Janosch - it says "plus minus plus". This is a tip for a minigame that will be activated when you knock on the door. The maximum amount of knocks in one turn is 5. You need to knock a given amount of times, count knocks in response, and depending on the turn, add or detract knocks according to the tips on the paper. For example, if two knocks are right, you'll hear a single knock in response. The amount (plus on the paper) of your knocks and guard knocks will amount to 3. Once you knock 3 times, you'll hear 3 knocks, and if now you need to detract, the difference will amount to 0 - you can be sure that the right amount of knocks isn't 2. It is also not 4, as the final amount of knocks would equal 6, where the maximum amount of knocks is 5. The good answer is 3 knocks. When you knock 3 times, you'll hear a single knock in response. Now, knock 4 times (3+1; plus on the paper). You'll hear 3 knocks, knock once (4-3; minus on the paper). The guard will respond with 4 knocks, you knock 5 times (1+4; plus on the paper). You'll be let in. Talk with Janosch and Rufus' ex-girlfriend, Toni.