I found something interesting on that. See:
[url]boot - Ubuntu 18.04 hangs on booting with message "started user manager for uid 120" on Asus 1015PX - Ask Ubuntu
That URL mentions editing “/etc/gdm3/custom.conf”. That particular fix involves uncommenting a line to disable Wayland. I see in my system this is already uncommented. Have you done anything with Wayland? On the system with the issue, can you check if “WaylandEnable=false” is uncommented (it should be uncommented)?
Note that wherever you flash from will have a “rootfs/etc/gdm3/custom.conf”, and if you don’t have direct access to the Jetson to check its file for that change, then you can use this file on the host side as an alternate check.
FYI, UID 120 is the user “gdm”, which the display manager used for GUI login.
Even if things seem to stop at that error message you might try CTRL-ALT-F2 or CTRL-ALT-F3 and see if a text-only console can be reached.