What is security_ack?
What is it for?
Can't find any docu about that.

No idea, nothing from Zenphoto itself. Where does this appear?

zp-data/zenphoto.cfg.php

I have no idea where this would come from. Never seen that and cannpt find a reference to this so far. It is for sure not part of the default Zenphoto config file.
https://github.com/zenphoto/zenphoto/blob/master/zp-core/zenphoto_cfg.txt

Try to remove it and see if it returns. The server log perhaps has some access to the config file if it is changed somehow.

Comparing my zenphoto_cfg.txt with github then:

I am missing line 77
'gallery'=>

and in line 92 and 93 I have:
$conf['security_ack'] = 2;
$conf['FILESYSTEM_CHARSET'] = "UTF-8";

I see it on our live site as well but none of my other (local) sites. Somehow I don't remember this at all, probably setup adds it on certain occasions.

Will take a look later.

Found it finally, it was a bit hidden and my IDE didn't search setup files because I had them protected….

So it is from a setup specific function named `acknowledge()`. It sets this if you decided to ignore these issues setup reported:
- `display errors` on
- `register globals` on

Both should be off generally.

Thanks for investigating!