Hi Pascal.
That’s a dependency cycle on the IK constraint.
The target for the IK cannot be connected to the IK chain at all. In that case it looks like the hand control is parented to the chain that it is to be affecting.
It’s a classic mistake, and when I first tried to learn rigging (YouTube didn’t even exist) I was trying to follow along with a PDF tutorial (yes a printed pdf)
I made this error and it freaked me out so much that I closed Blender, turned off my computer and left the house because I needed to get away from the broken mess I just made. I’m not even kidding.
Thankfully I watched a Bassam Kurdali tutorial later (on DVD….yes DVD) which explained this in a way that made sense to me.
