I finally received the 3rd part of this quest, but I've had a bit of a mishap.

I have my main island where the quest giver is, and I am allied to the Corsairs. When I received the quest to end the blockade, I heard battle music and then it disappeared quickly.

No ships were spawned so I can't do the quest.

Anyone else have this problem, or know how to fix it?

I had exactly the same issue a few months back mate. Basically the quest-line is broken. Save yourself some time and read this: I contacted Anno support and Ubisoft .. neither were able to help me with it. Ubisoft said they were passing it to another department and my ticket was marked as 'solved' .. the Anno support refered me to a forum that didn't feature 1404 at all, and then suggested a walkthrough .. fail. It's this kind've thing that's let the game down in a lot of ways, especially as the questline is part of a meta-achievement.

The only way to do part 3 and then part 4 is by using a mod which will also disable achievements .. so in other words, not much of a work around.

Sorry it's not good news bud, but at least I saved you a week of frustrating emails and empty promises.

Bah..

That's really disappointing, well thank you anyways.