MacBook won't login, nor start in Safe mode or single-user mode.

I bought my MacBook some months ago and since then I am having kernel panics, probably due to a hardware issue, related to the memory. Those Kernel Panics usually make the laptop restart and everything work after. However, yesterday it crashed and probably it was writing something important, which got corrupted. Now, the laptop is not letting me login anymore and startup.

The laptop startsup and shows the login screen. When I try to login, at 3/4 of the loading bar, it suddenly shuts down. It is worth mentioning that I have FileVault activated but not the firmware password. Also, I have no time machine backups due to this issue The backup disk needs 3.84 EB for the backup but only 5.44 TB are available. I already filled.

I tried many things to recover the normal functionality and be able to back up my data:

• Safe mode: holding Shift won't change the output, it just boots to the normal login page, and the login does not work yet.
• Safe mode + Verbose: I can see it crashes throwing this error: BuildUser(): error building a user of type 0x20010008.

Single-user mode: I get the same error: BuildUser(): error building a user of type 0x20010008.

Recovery Mode: I can boot in recovery mode, and this is probably the way I will use to backup my data, using the terminal.

This topic describes my same issue: MacBook Pro El Capitan Freezes on Boot Up.

After I have backed up the data, I plan on sending the MacBook to Apple for reparation (because of the Kernel Panics issue).

Though, as an engineer, I would like to understand what is going on and why is it broken. Could someone guide me on how to debug the login/startup process of OSX? I tried to check /var/log/system.log but it feels like there is nothing useful there.

Thank you,

Carlos.