Comment 10 for bug 1615021

I don't actually know what BOOT_DEBUG does -- I've never seen it before, it does not appear anywhere in my yakkety system, and it's for sure not something the kernel, initramfs-tools, or systemd look at. My best guess is that this is a debian-installer specific debug flag.

So from what I can tell, the readlink path issue is merely a red herring -- it's good to fix it of course, but it's unrelated to the boot failure.

Since this is a heisenbug, it rather seems to me that this is some timing issue -- any extra debugging, or time spent with changing boot parameters in the boot loader will change the behaviour (e. g. make the detection of network devices by the hardware finish earlier).

ATM I'm afraid there isn't enough useful information here yet to understand what's going on -- indeed having a screen output where the problem does happen would be helpful. dmesg logs and "udevadm info -e" as well, as Steve says.