DE10-Standard U-Boot trouble

Hello everyone,

I’m having a bit of trouble getting the DE10-Standard to boot using U-Boot.
If I use the standard SD-Image from Terasic it boots just fine, even if I changed the Linux kernel and rootfs to more modern versions.
The trouble starts if I want to use my project. I followed the tutorial from BuildingBootloader to the letter, using the source code from the linked repositories. It doesn’t matter if I use the rootfs build by yocto or a bootstrapped Debian or Ubuntu.
I always get the error message “Trying to boot from MMC1 / Card did not respond to voltage select! : 110 / spl: mmc init failed with error: -95 / SPL: failed to boot from all boot devices”
Same message if I build U-Boot from their own repository using the defconfig for the DE-10 Standard.
If I simply use the GHRD image from Terasic and add my rbf for the FPGA to boot, it shows the errors:
Unable to read file soc_system.rbf
reading zImage
reading socfpga.dtb
Bad Linux zImage magic!

What am I doing wrong in getting my project loaded on the board?

Thank you all in advance for reading and your solutions!