If some other people get stuck with the problem, I came with a backup solution. The problem is to keep objects moving in diagonal but also to not find path between two close diagonal tiles.
Use the Pathfinding behavior with diagonals off, only to find a path and avoid setting path between two diagonal tiles. And for the movement, use another behavior and regularly test if the next node on the path is in line of sight or directly reachable via raycasting or whatever. If so, then set the object to follow on with this next node.