Object location jumps during weight paint

I rigged a hand and animated the bones. The hand animation works nicely. The problem is that I want to move a separate object along with the hand movement. So I parented this separate object also to the hand armature. When I now go to weight paint, everything looks normal at first. But as soon as I start painting the object with red, it jumps to a different location. What could I be doing wrong here? Is the frame I was on when creating the parent possibly messing this up? I did the parenting after the animation has stopped moving, because then the mesh lines up with the hand.

Any ideas are welcome!