Please try to login via su:
sudo su - openhab -s /bin/bash
tryout another sudo command like
openhab@openhab:~$ sudo ls -l
which should result in something like
We trust you have received the usual lecture from the local System
Administrator. It usually boils down to these three things:
#1) Respect the privacy of others.
#2) Think before you type.
#3) With great power comes great responsibility.
[sudo] password for openhab:
then escape and retry (now only the question for password should emerge). escape again.
now tryout systemctl reboot