Where did things go wrong?

I guess you missed a particularity of Frisian rule: if a player has equal capturing options with both simple draughtsman or a king, a player is obliged to make capturing move with a king.
18. .. 49-21?? was the move that turned your winning position into a loss! Yes, you deserved a win!
The correct winning idea in similar cases is attacking multiple draughtsmen which are located on central squares.
In your game black could deliver a winning sacrifice:
18. .. 14-20 19. 25x23 (white needs to take two black draughtsmen, but not one!) 49-35,
and white has no opportunities to escape from losing three draughtsmen.
If, for example, 20. 12-7, black should not worry that white goes for crowning: 20. .. 35(x19)x24, and white can't crown 21. 7-2 due to 21. .. 1x3.