is there a "manual" way to reset a site to "online" status ?
(e.g. a DB field or what - on a test ground, it cannot remove the "offline" status, I would like to check if al is ok. ty.

Yes, find the setting$conf['site_upgrade_state'] = "closed" in the config file and change it to $conf['site_upgrade_state'] = "open";

It also allows the value "closed_for_test" so being loggedin as full admin you can test the site while it still being closed for all visitors.

But normally being loggedin as admin you should be able to change the maintenance mode that right from the backend. There is even a bar above on the backend and wihtin the admin tool bar on the front end indicating any closed status.

In case you missed the new behaviour while running setup is documented in the 1.6. release post in the section "New utilities": https://www.zenphoto.org/news/zenphoto-1.6/#--new-utilities and on https://www.zenphoto.org/news/installation-and-upgrading/

ty will do that