I deleted the scene from the download location. I think I managed to find out a solution to my issue. Implies some masking, blurring, blending and such :).
The videos are 6K and 4K (but I trimmed them down from the original – only 40 frames). in total the files are 2.7 GB. As you see, I applied the Keylight and others (Key Cleaner and Spill Suppressor) on a log footage – Scene 7-debugging.mov – (later on I found out that is better to convert to rec 709, do the keying and they eventually use the mattes to key the log footage). Then after I keyed I applied some color correction to the resulting footage. Then i added a background video – PL40105-1274-07-background-debugging.mov. Also, as you see, I tried to do the keying in several steps, masking parts of the video and applying Keylight with a bit different settings to each of it. The hand I am trying to get it right also has blur from movement.