Today I received a new mainboard from creality, and my problems are gone! Look like your mainboard is fried.

I were in contact with the support, they send me a new mainboard.
Now i am waiting for it, to test it, I will give you feedback when i got it!

I got a new Ender 6, but it has some problems i can not solve.
The first prob ist, it will only start up when the SD-Card reader board is not attched. It will also run, when the board is attched, but not screwed on the housing.
The second problem: The temperatures are both -15°. I measured 100k ohm at the cables the plug into the mainboard, so everything ist wired correct.

I think its a problem with the mainboard, what can i do?