As someone pointed out in another thread the problem is not with Witcher 3, but with the latest version of GOG Galaxy. As of right now no achievements seem to be working in GOG Galaxy no matter the game. I am certain an update (and a solution) will be available soon.

Ok, thanks for the tip. I assumed it was a BaW only issue since achievements worked the very day before I started playing the expansion less than a week ago, and this thread is from June 1st ...

GOG are claiming that the problem has now been fixed. Can you get achievements to work now? Otherwise the problem is with the game (or they have yet to solve it entirely).

Yep, they seem to work now, but sadly not retroactively :( so I hope you guys have some hard saves readily available

It seems they´re working again, but please make it retroactive. At least for achievements that are tied to the quest completion.

Guys, all works.