Thanks -- reverting the Kyll river fixed most of the damage. There was also a creek ("Katzengraben") that looked very chaotic. I removed the random-looking parts of it and attached the rest of it to the Kyll river. Apparently that was what the mapper inteded.

The area looks alright again, but a slight misgiving on my part remains. Is there a tool that graphically displays "Before" and "After" of a Changeset? -- Regards, Raimund