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Many thanks to everyone for all the input - I didn't expect so many replies in such a short timeframe. I need to take some time now to carefully study all the suggested solutions - it's all very valuable info, like a textbook Special thanks goes to...
Dear Kurt, Thanks for making a minimum version that shows the same unexpected behavior. What the exact issue is I don't know. Maybe it tries to triangulize the points (by default a nice n*m grid of vectors is shown). Maybe the triangulation fails...