Hello, I'm new here and new to armbian,too.
I have exactly same error. Clean install of 5.21 and I can reboot and shutdown any times. After running apt-get upgrade I can restart once, the second time it hangs.
I've noticed this error during upgrade:
ln: failed to create hard link ‘/boot/initrd.img-3.14.79-odroidc2.dpkg-bak’ => ‘/boot/initrd.img-3.14.79-odroidc2’: Operation not permittedContents of the boot partition:
And the complete serial console log: