This rather looks like an issue with the DTB file to me since that's what the "loading fdt" is about. Did you try to replace the dtb file with a version from an earlier kernel? Since there are two rockpro64 dtbs that are linked - did you make sure you have both in your dtb directory?
Afair, I used kernel 5.6 and later 5.7 when writing my guide but I didn't use or upgrade my rockpro64 since then (I use the installation on other boards as I find the rockpro64 too power-hungry).
Edit: Double-checked - I can confirm the current kernel in Debian/Devuan unstable is 5.9 now, I will try to see how it works for me.
Edit 2: Kernel 5.9 boots fine (on the pinebook pro), however I have issues with the sdcard. Not sure whether this might be a hardware issue though.
Afair, I used kernel 5.6 and later 5.7 when writing my guide but I didn't use or upgrade my rockpro64 since then (I use the installation on other boards as I find the rockpro64 too power-hungry).
Edit: Double-checked - I can confirm the current kernel in Debian/Devuan unstable is 5.9 now, I will try to see how it works for me.
Edit 2: Kernel 5.9 boots fine (on the pinebook pro), however I have issues with the sdcard. Not sure whether this might be a hardware issue though.