After uploading the 1.5.9 version, I am unable to get to the admin page or run the setup (500 error)

If I go to my webpage/zenphoto I get the following message.

Zenphoto has detected a change in your installation.
The script template-functions.php has changed.
The script lib-auth.php has changed.
The script lib-utf8.php has changed.
The script functions.php has changed.
The script functions-basic.php has changed.
The script functions-controller.php has changed.
The script functions-image.php has changed.
Zenphoto 1.5.9 has been copied over 1.5.7.
The script SIGNATURE_HASH has changed.
The change detected is critical. You must run setup for your site to function.

How do I fix this?

Did you run setup after uploading the new version? It should have done that automatically but anyway.

And as always: Errors in the logs and which?