I'm not trying to log in as full admin. I'm testing the gallery password to see how the site appears to guests with whom I share the password.
To answer your questions, my ZenPhoto installation is located at http://mysite/photos/. I'm running the Stopdesign theme, but I have the same problem with the Default theme. I don't see any error log, just a debug_log.txt, setup_log.txt, and security_log.txt in the zp-data folder. The latter records the following when I log on with the gallery password:
[Date] [Time] [IP] Admin login Failed gallery-password
[Date] [Time] [IP] Guest login Success zp_gallery
which seems reasonable.
The password_form.php file isn't there any more. It was removed upon upgrade when I gave the setup script permission to delete unnecessary files.
I recently discovered one piece of info that could help pinpoint the problem. When I log in at http://mysite/photos/, I get redirected to http://mysite/photos//photos, which returns an "Object not found" error. If I then navigate to http://mysite/photos/, I see the gallery, so the log in credentials are being accepted. If, instead, I log in at http://mysite/photos/index.php, then the log in works fine (I see the gallery), and I don't get redirected. Perhaps somebody who knows the code will know where to look? Thanks!