16.04.3 LTS, CLI login loop, how to exit?

[JusticeEmpire]

I've had a look around the net, and although I can find answers to similar problems, I can't find an answer to the one I'm facing.
I have no GUI/Desktop/Space issues, it is a standalone server running on CLI only.
When I try to login, it logs me in then flashes some text so quickly I can't see, then returns me to the login prompt.
I filmed it, paused at the relevant time, and can now see that it logs me in, displays the updates, and then says there's a new release available and to run 'do-release-upgrade', before returning me to the login prompt.
Any ideas or solutions?
Apologies in advance if this has already been asked, but I couldn't see it anywhere.

[JusticeEmpire]

Since then, I've done a hard shutdown, and it is now giving me the same problem as this question, however, I am trying to boot off a USB as my OS is installed on a SD card.

I'll continue searching, but does anyone have a quick fix for that?