Thank you jerome. Now that you answered it seems simple - I should have figured it out on my own!
I am trying to run the Hough-based ellipse detection. It works fine with the image data.coins, but fails with my own images. I suspected, that the “number in brackets when opening the image” was the reason - but cropping shouldn’t have an impact on the function (only shorter runtime).
I posted about this issue before, but nobody answered - probably I provided too much information at the time.
Any idea, how I could solve the problem?