Hi sn2018,
please use the web-ui and activate SSH for user pi as described in
(Its currently possible via Web-UI only!!)
This will activate the user pi again (which is part of sudoers)
Afterwards you can logon with user pi from terminal/console (physically keyboard/display connected to your device) as well as using ssh.
Crazy workaround, however the only one i am aware of for now.
Best regards
schoetju