After some serious GRUB issues after routine kernel updates, forcing a grub-reinstall via chroot and some single-user mode maintenance, I now cannot login to my server (except via single-user mode)

When attempting to SSH to the server my connection is reset immediately after entering username - no password prompt is presented.

I've plugged in my monitor and keyboard to the headless server and when trying to login I receive: login: "PAM Failure, aborting: Critical error - immediate abort" immediately after entering my username - again, no password prompt is displayed.

I am able to gain access via kernel recovery mode and login as root through single-user mode with networking. I've attempted a dpkg reconfigure believing I may have had some incomplete/borked packages, but to no avail.

Looking through /etc/pam.d/login doesn't seem to lead me in any obvious directions and I'm afraid I'm out of ideas.

Googling doesn't help me much, one guy reinstalled (really don't want to do this) and the rest I could find were old Gentoo related bugs.

Any tips?