Thanks, that worked. Before your answer, I clicked on each tile to choose it as the next one, now I see that dragging the mouse over the grid will also work and solves the crossing problem. Strangely, the click-solution worked with the level where you have to cross your same-colored track.
Nice game, I usually don't ask for clarifications if I'm not interested. I did study the website, and I didn't find "dragging", ist just says "LMB". For me that means clicking, and it does work for 3 levels, so it becomes hard to understand that clicking is more of an "accidentally" working mode.
But interface design is complicated, again, the game looks and plays fantastically, I'll play more of it later.