For me, this is an SD card issue. I attached the UART pins to my PC and the bootloader fails with messages about the SD card not working. I've recompiled my kernel for the Odroid C4 and reversing some of the changes as suggested by @Cornelius fixes the reboot problem. The C4 reboot hander needs the SD card reset code to reset voltages for UHS-I cards, so that they can be detected properly by the bootloader. The patch can't be simply reversed because some of the upstream power management code has been changed.
I will try and get my patches merged into the mainline kernel, but in the meantime I'll submit something to the Armbian project.
Changing to a different SD card (A1 perhaps, or non UHS-I?) or to an eMMC card will likely solve this problem, but of course they might cost money!
