Thanks for the fast help!
These settings did the trick. Frankly, I didn't expect them to have any effect, when offsetting edges made of exact circles and lines. Naively, I assumed this to happen with within numerical precision
Well, intuition sometimes fails...

When inspecting the corners of a simple profile operation, I recognized some strange circular movements. They appear with cutter radius bigger, equal and smaller than the corner radius. I tried V0.18.4, daily and my own compilation of the most recent git version - all the same. To my understanding, ...

Tried to write some bug report. However, I always got a message the FreeCAD information field was empty. Still, it wasn't empy as I pasted the information directly copied from FreeCAD. I also filled in all the other fields marked as mandatory. What else could be wrong?