"You have been dismissed from both the party and the duty." Is this a bug?

[Vahlnir]

Also this never happened before and now I got it twice this is why I'm wondering if it is some kind of bug that breaks me from the party.

Typically, once if a fluke. But I find it highly unlikely that you simply found 2 trolls that happened to have it out for you for no reason. There's more to this story I think unless it is indeed a bug. Anything's possible.