Okay, I had an old installation of ZP (1.1.6) and I figured it was time to upgrade. I downloaded 1.3 yesterday and uploaded it to my own server. However, I'm hitting an error in the installation script, during the Systems Check; after validating the MySQL connection, it moves on to checking zp-config.php but fails with the following error:

Fatal error: Call to undefined function zp_loggedin() in /home/www/vhosts/something.net/httpdocs/zenphoto/zp-core/setup.php on line 879

Any hints?

Try a re-install/re-upload. Probably something went wrong with that. Be sure to follow our upgrade instructions.

Interesting. I have deleted and re-uploaded all of the folders from the installer, but when I run setup.php I get a new error now (the only thing on the page):

Zenphoto encountered an error MySQL Query ( SELECT * FROM [code]zp_administrators[/code] WHERE [code]valid[/code] = '1' ) failed. MySQL returned the error Unknown column 'valid' in 'where clause'
I hope this hasn't b0rk my database...

Please read here: http://www.zenphoto.org/support/topic.php?id=7472#post-43899
It is a know problem from upgrading from old releases. I just had to search myself.