Hello, thanks for your feedback...
I was able to solve this problem successfully. Thanks to this post from pbasista:
TFTP download from TP-Link Archer C2 does not work properly - #15 by pbasista true (-> Thanks pbasista!) . I was able to overwrite the wrong uboot version (C20) with the C2 via the serial connection. Now everything works perfectly... Thanks to everyone involved for this great software