Yeees, I did it 
That was quite a challenge!!! After fixing all the bugs I could find I’ve reached 7th in the gold league after that I decided to play with the exploration parameter (sqrt(2.0) - 1.0) looked like working the best, submit -> 1st in gold league for a couple of days I could not think of anything else then I saw that I haven’t almost any battle ended in a draw. I updated my algorithm with 0.5 reward for draw(0 for lost, 1 for win), submit -> again 1st in gold…, but I saw less losses and more draws. After playing for several hours with the exploration parameter 0.5 value really balanced the search, submit -> 80th in Legend!!! And I felt really really happy.
So in the end it turned out, as @darkhorse64 said, that the exploration parameter must be adjusted properly, also the pruning proposed by @accorp guided the search in the correct direction.
From this point on, I think the main improvement for my strategy would be to optimize the C++ code, I have an idea how to make it about 2 times faster (but maybe in another parallel universe) or improve the selection phase (I read a ton of literature on the subject).
Thanks to everyone for the suggestions. I wanted to learn MCTS with this game and I think I did.
Great game, great platform, great community and it feels great to see the result of the efforts made!