Hi Gunter, the issue is that the surface you are targeting there is a trimmed surface. That means that there is an "underlying surface" that is larger than the visible active area and there are trim boundaries on the surface to mark the active area. That larger underlying surface is what is used by the Flow command.

You can see the underlying surface better if you select the target face and use Edit > Separate on it to break it apart so it's a single surface not joined to any others and then type the Tab key to put keyboard focus into the XYZ input control on the bottom toolbar and then type ShrinkTrimmedSrf and then the Enter key.

Then use Edit > Show pts on that surface and you should see this:



That's the actual surface that will be used for the Flow operation.

You basically can't use booleans or trims to try and reshape the flow, you need to generate a full untrimmed surface for your target.

- Michael