Hi Heiner, it looks like the problem in your case there is that you've probably enabled the "Projective" option for Flow, which is a checkbox that shows up in the pick target object stage, here:



In order for your case to work properly and to follow along with the steps I had earlier, make sure that checkbox is unchecked. If it is checked then that will enable a different mode for Flow which is called projective mode, and in projective mode it is expected that every ray that shoots out from the base plane should intersect the target object which is not the kind of arrangement you have.

Projective mode is more for beaming on a small shape onto a local area of an object, rather than stretching it all the way across a target surface. For your case here you want the regular "full stretch across surfaces" method so you will want that checkbox turned off. It is off by default for the start of a program session however if you have enabled it during some previous test of using Flow within this same modeling session it will remember the previous state and so that's probably how you ended up with it turned on.

Anyway, if you uncheck that option you should be able to get things to work as in those previously described steps.

- Michael