Thanks for the reply's everyone,
I did some troubleshooting, but could not find the exact reason why it trips the GFI. I did not want to keep it this way because my domoticasystem resets everytime the GFI trips, so in the end I opted to change my whole setup. As the simpleEVSE board, relay, wires etc. in the charging handle looked quite flimsy, I also didnt fully trust that setup to last long and endure a lot of abuse before something breaks or short circuits.
Luckily EVracing also send me a EVSE W16 board (not sure why, they labeled it 'spare parts'), which has the evsechip,12vdc adapter and 2x relay all on one small board soldered. I printed a small 3d casing which exactly holds that board and a push-button and put it in the powerline, looks very sturdy and waterproof (not planning on testing that though).
I complete emptied my bike and gave it a full charge with the cable without problems, even tried 20Amps for an hour (you can adjust that with the button) to see how warm everything would get and if it trips the breaker:) it did get warm but nothing serious, and no breaker tripped. also the powerline from schuko into the house held up nicely (2.5mm2)
So, i will keep testing and using this cable, let's see how it holds up longterm. For now it looks like i am able to give my bike a full charge in about 3 hours at home:)
For anyone interested to copy this:
-EVSE W16:
http://evracing.cz/products-3d model in attachment
-cable between plug and 3d-case: ÖLFLEX® FD CLASSIC 810 7 G 1.50 mm²
-cable between 3d-case and schuko: 5m BREMAXX AT-N07V3V3-F 3G1.5
Cheers! David