Found this: HPB Down, nothing helps - #3 by sven1234
sudo bash -c 'source /usr/local/etc/library.sh && clear_opcache'
sudo service php8.1-fpm restart
This did replace the service unavailable screen with the into screen that tells me NCP is in maintenance mode.
And today it really was in maintenance mode (I saw the checkmark in the panel).
After disabling the maintenance mode my NCP now is running again!