I am trying to install zenphoto in a sub-domain of my website. The files were uploaded to a folder zenphoto and the sub-domain gallery.rhyniecharitabletrust.co.uk pointed to this folder.
Entering that url gave an error message: 'The site is undergoing an upgrade. Please return later'
So I tried gallery.rhyniecharitabletrust.co.uk/zp-core/setup.php and just get a blank page.
Can anyone help please ?

The (new) "closed for updata" behaviour is explained on the installation page: https://www.zenphoto.org/news/installation-and-upgrading/

Did you complete setup at all? Did you perhaps enter wrong credentials for the db? Access /zp-data/on our install and remove zenphoto.cfg.php. Then re-run setup.

Or enter the correct database credentials within that file. Then setup should run again.

The set up didn't run at all, so I had not entered any db credentials.
All fine now, I think.

If it was a fresh install setup should have started if you entered the url to the site/subdomain itself.

Setup directly does not work becauase you need to be logged in if it is not a fresh install. But a blank page mostly means some issue that should be in the debug log.

Thanks @acrylian