Thanks for reacting.
The code calculates the crossing point(s) of two circles. They are two distances point E must adhere to.
It works as expected and places object E (named axle_E) at the right location.
This location is where the head of bone Bone_F should be. Is working fine.
I want bone Bone_F2 to be exactly 90 degrees onto bone Bone_F.
When I rotate bone Bone_F with Track To disabled, bone Bone_F2 follows Bone_F correctly as I want.
But when I enable Track To for Bone_F, is seems to flip constantly.

Yes, it is still distorted, even in Blender 2.90.0 Alpha, up to frame 200

How do I do that?
I enabled and disabled each and every choice in the filter-tab.
No markers visible in both Dopesheet and Timeline.
They are visible however in the Graph Editor.
Selecting and deselecting the bone doesn't help either.
Really stuck here.

It's not clear for a beginner like me that you have to select Bone in the bone-textfield. In my opinion it is obvious that a bone is a bone.
Your comment was needed badly to nail problem 1.
Thanks.
Problem 2 is inaccuracy of Blender somewhere.

Changing anything in the filter has no effect;
I still don't see any key-info in the dope-sheet.

I switched over to Non-lineair Animation, noticed stopping of the sound at frame 900, changed Frame End in Output Properties to 1800 and noticed a short block at track SoundTrack, moved it to position 0 (there also was no sound till the cursor reached this block), et voilá, sound. But distorted initially. Why?

You are the life-saving Master!

Thanks for the reply.

See the exact steps

Thanks for the quick response.
You're right, my beginners mistake.

Memory is at 56%
Same problem with both Experimental and Supported rendering.

Now (2.82a) is works as you are explaining.
Thnx for your attention anyway.
I'm grateful.

Adding a keyframe makes no difference.
Changing start- and end-frames to 1 and 100 does the job.
Animation only works within the range of 0 to 100.

I don't see how to start a Task Manager in the manual.
Please inform me.