On dreamhost, the setup run when upgrading to 1.4.4.5 consistently gets a 500 error. (While I'm doing an upgrade, it's a test installation that I don't have much invested in, so just trashing it and putting in a clean install is a good option for me right now. On the other hand, if the testing convinces me I want to use Zenphoto regularly, then I'll start being very concerned about upgrades....)

The error log just says "[Sat May 25 15:58:56 2013] [error] [client 216.243.177.140] Premature end of script headers: index.php, referer: http://dd-b.net/zenphoto/zp-core/setup/index.php?autorun=gallery", doesn't give me any ideas.

Look also in the setup log in the zp-data folder. Perhaps there is more information there. Otherwise, if that is the only error in your logs it really has not told us anything.

One possible cause is things like file permissions and folder permissions. Many hosts will error-out if the permissions of script files (folders) are not to their liking. Unfortunately, "their liking" is not consistent among hosts so I cannot tell you what dreamhost wants.

It is possible that dreamhost provides a SimpleScripts install program for Zenphoto. We know that current SimpleScripts versions properly install Zenphoto and presumably insure the correct file/folder permissions. If they have that setup, give it a try. If it is for an older version (unfortunately often the case) you can always install 1.4.4.5 over what it installs.

More info -- I tried a clean install in another directory and ran into the exact same error. zp-data/setup.log shows nothing but the config information I was shown on my screen before I clicked go. After a minute or two, setup died with a 500 error same as before.

I've got a screen capture of the config screen, though nothing on it looks suspicious (warnings on software versions, but no errors; these are the versions provided at Dreamhost, so I don't have any choice). I don't see an option to attach files or embed an image here, though.

I didn't see anything interesting in the setup log, but here it is:

Zenphoto Setup v1.4.4.5[578af0f46b]: Sat, 25 May 2013 17:46:43 -0700
Warn: Log security [is compromised] Zenphoto attempts to make log files accessable by owner only (permissions = 0600). This attempt has failed. The log file permissions are 0660 which may allow unauthorized access.
Warn: PHP version 5.2.17 PHP Version 5.2 or greater is required. Version 5.4 or greater is strongly recommended. Use earlier versions at your own risk.
Warn: PHP display_errors [is enabled] This setting may result in PHP error messages being displayed on WEB pages. These displays may contain sensitive information about your site.
Warn: The Zenphoto filesystem character define is UTF‑8 [no test performed] Setup did not perform a test of the filesystem character set. You can cause setup to test for a proper definition by creating a file in your zp-data folder named charset_tést and re-running setup.If your server filesystem character set is different from UTF‑8 and you create album or image filenames names containing characters with diacritical marks you may have problems with these objects.Change the filesystem character set define to Unknown ASMO-708 BIG5 CISO2022JP CP1026 CP870 DOS-720 DOS-862 EBCDIC-CP-US EUC-CN EUC-JP EUC-KR GB2312 HZ-GB-2312 IBM437 IBM737 IBM775 IBM850 IBM852 IBM857 IBM861 IBM869 ISO-2022-JP ISO-2022-KR ISO-8859-1 ISO-8859-15 ISO-8859-2 ISO-8859-3 ISO-8859-4 ISO-8859-5 ISO-8859-6 ISO-8859-7 ISO-8859-8 ISO-8859-8-i ISO-8859-9 JOHAB KOi8-R KOi8-U KS_C_5601-1987 MACINTOSH SHIFT_JIS UNICODE UNICODEFFFE US-ASCII UTF-7 UTF-8 WINDOWS-1250 WINDOWS-1251WINDOWS-1252 WINDOWS-1253 WINDOWS-1254 WINDOWS-1255 WINDOWS-1256 WINDOWS-1257WINDOWS-1258 WINDOWS-874 cp866apply
Warn: MySQL version 5.1.53 MySQL Version 5.0.0 or greater is required. Version 5.5.0 or greater is preferred. Use a lower version at your own risk.
Completed system check

There may also be a debug log. If so, what is in it?

Seems you are getting some sort of a time-out. Waiting a minute or so for the next setup screen is not normal. What is the URL in the browser address bar? It should look something like //zp-core/setup/index.php?checked&update&mod_rewrite=ON.

You might try that link directly to see what happens. Also check your MySQL logs. Maybe something is going wrong with the database.