You made everything on the machine owned by root. Users home, some logs, and other config files are not suppose to be owned by root.
I cannot access the server through the Display Manager
I assume that you are doing that as user?
Could not update ICEauthority file /var/lib/gdm3/.ICE authority
Because the file is owned by root, or does not exist.
Usually root logons with ssh and xserver are disabled by default.
Are you trying to access the server as user or root?
Is there a ~/.ICEauthority? Change ownership back to user.