Ok heres a clip that came out a while ago. 

(just ignore the beginning and the end of the clip which has videos for the Green house academy  and everything sucks)

Ok, lets break it up...

First we see hiccup in the alley way at the northern markets, he is with toothless and he has his sword out. You can just see Trader Johan in the background.

Then you see a clip of the dragon flyers flying towards the northern markets.

Now I'm going to try and fit this into the most logical story line I can think of.

Trader Johan called hiccup to come quickly to the Northen markets, Johan is setting a trap for hiccup. Hiccup comes alone to the markets and the dragon flyers come, Hiccup then trys to protect Johan and then somehow finds out that he is evil. Hiccup is probably taken prisoner and Astrid and the rest of the gang need to break him out. 

(I think these clips where from early in the season since he does not have is HTTYD2 sword yet) 

What do you think? Plz tell me if there is clips, photos or stuff I'm missing!!

Update: I just found a better clip on Instagram:

https://www.instagram.com/p/BeV1EbaneuY/?tagged=rtteseason6

It is exactly the same but at the beginning it has dragon hunters shooting a Hiccup and toothless.