OK, good news.
Tried it from scratch and it just works on
Unity 2017.1.0f3, Playmaker 1.8.5.f8
Just one show ad action and it works. Easy!
Now off to figure out what can be the problem in my project...
I'll post here when I manage to figure out what the problem was, so others don't fall into the same trap.
br,
szomaza