The Image I tried is "Armbian_5.38_Nanopiduo_Debian_stretch_next_4.14.15.img". The issue with U-Boot was probably because of a defective card reader which I replaced now. Now every time I dd an image I check it with cmp afterwards. So this way I can be sure the SD-Card has the correct content. I have 2 Cards, a noname one with 16GB and a 32GB Sandisk. The 16GB one freezes at some time with no error: The 32GB one gives strange filesystem errors: badblocks shows no errors on both. Using my custom created Image I have no problems except with SPI.
I cannot get SPI working on my Nanopi Duo or rather there is no /dev/spi*. I build a custom Kernel with the build process and Network and other stuff works. Loading the spi-spidev overlay in various ways did not help. (Loading via U-Boot, during runtime, combine the overlay and devicetree and compile it as whole) By the way, the Images from https://www.armbian.com/nanopi-duo/ are not even booting through U-Boot for me. Some Infos: