I removed Magic Lantern from the firmware menu (waited 30sec to delete the bootflag).
Now the camera behaves the same way I got it from the store.
I dont wait for any LEDs, and no matter how fast I remove and insert the card, the camera turns on without a problem. I can even remove the card without turning off the camera - it still works.
So that means ML needs some time to initialize and if the user moves the card too quickly, the camera bricks and needs battery re-insertion.
I am still wondering, how is it technically possible, to drain a fully charged battery in just seconds? What process could lead to this?