I had a similar issue, reboot did not work with Armbian_20.02.0-rc0_Odroidxu4_buster_current_5.4.11_minimal. Updating the boot loader had no effect.
Since I have no additional Micro SDs available, I tried to change the image to Armbian_20.02.0-rc0_Odroidxu4_buster_legacy_4.14.165. That worked.