Oh guys, many thanks for your professional help!
I am at work now and can't wait to come home and try it out.
I measured with a multimeter before I started soldering and must say that the pins of my microSD->SD Adapter are not 1:1. I mean the SD Card contacts row with the microSD contacts row.
So SD: 9,1,2,3,4,5,6,7,8 is not the same at microSD.
But okay, if you say my wiring is wrong, I'll believe that and will try it with your wiring order.
Is this connection order the right one? (Found in a chinese tutorial):
PS.: I don't want to flash a NAND backup because I don't have one. That german tutorial says that actually it is possible to unbrick the 3DS w/o a NAND Backup needed