First, you can check ubiboot log file to see if it goes successfully until executing the kernel used by Debian. If ubiboot is happy, you should check Debian syslog logs to see if there are any logs about this boot attempt. If not, it usually means that either the kernel didn't boot successfully, or userspace has crashed very early, and this makes debugging much harder. If logs exist, you should check them to see what went wrong. BTW, remember that in Debian, you should have started kicking watchdog files from early in the boot process (usually, both in its initrd/initramfs, and after continuing the boot process by some simple service.