Well here’s the page in the documentation.
https://docs.unrealengine.com/latest/INT/Engine/Animation/RetargetingDifferentSkeletons/index.html
Those strange artifacting problems can often be fixed by viewing the skeleton, pressing the button to show advanced and right clicking on the final bone in each finger to select “recursively setting translation retargeting to the Skeleton”. Here’s a youtube tutorial showing the technique being used if you’d prefer a visual example.
(between 13:40 and 15:40 on the video)
Not sure if that’s the specific solution to your issue, but hope that it may help.