[SOLVED] Disable graphical interface.

Try this as sudo user (and perhaps test it first in a non-critical set-up):

systemctl get-default
This should show you the current setting, i.e. graphical.target

Then confirm that the terminal mode is available (it's called multi-user.target) in the list of target files:
systemctl list-unit-files --type=target

If you have multi-user.target in the list of target files you can proceed with setting it as the new default target:
systemctl set-default multi-user.target
If you now reboot the system it will boot into terminal mode, i.e. it will give you a shell with login prompt instead of the desktop

You can of course get back to the graphical mode by
systemctl set-default graphical.target and reboot