Hi Tessa,
Welcome to RP!
Thank you Flora!
And thanks both of you for your tips, but none of them worked. I even tried the extra layer before I posted too, to be sure, but then the clone stamp behaved as an ordinary cloning tool, even though I set "sample all layers" and "Color" in the option bar.
But...then I tried today again on a different picture and it worked out just fine. Even the clone stamp tool and an extra layer behaved as it should with these settings, but still I can't make it work on the picture enclosed!
Bugs? I really don't know...