Actually I didn't find any mistakes. The problem was that the power outlet had poor mains earth, combined with the fact that I put the device on my workbench with no ground plane.
The subsequent tests are done umsing a power outlet, where I trust the mains earth.
For good secondary, two important things:
1. No loose wires inside the coil!! Make sure to shorten the wires connecting the ends as much as possible.
2. Many coats of polyurethan (or similar). I made a jig to ease the application using an old powerdrill connected to my bench powersupply. After applying each layer, leave the jig rotating slowly to ensure even coating. At least 5 layers.
https://www.instagram.com/p/BFY37eckNaU/