Thank you for the simplified project. The first thing we will do is delete everything that doesn't affect the problem, so I appreciate that you have done that.
The problem is here, the constraint [FIX]F-arm:
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When scale X and scale Y are different values, it is called non-uniform scale. This constraint applies non-uniform scale to bone F-arm. The bone F-wrist is a child so it inherits scale from its parent. That scale from the parent is applied in the direction of the parent. When F-wrist points in the same direction as the parent, F-wrist is scaled shorter. When F-wrist is rotated 90 degrees from the parent, it is no longer shorter, it is instead made wider. As your IK target is moved, F-wrist is rotated and the scale from the parent changes its length.
To fix it, use uniform scale, where scale X and scale Y have the same values:
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With this, no matter what rotation F-wrist has, it will always be the same length.
You probably want to use uniform scale in other areas. I also suggest turning off transform constraint mixes you don't need, eg rotation and shear.
I forgot to mention, my earlier project was saved with 4.2, so it won't open in 4.1. It's probably not useful now anyway.
