After closing the site to upgrade from 1.4.5.1 and just run the setup scripts on the same files to test them, I am redirected to https://mysite.com/zenphoto/plugins/site_upgrade/plugins/site_upgrade/plugins/site_upgrade/plugins/site_upgrade/plugins/site_upgrade/plugins/site_upgrade/plugins/site_upgrade/plugins/site_upgrade/plugins/site_upgrade/plugins/site_upgrade/closed.php
What might be going on here?

Your site is closed in Maintenance mode (see Upgrading #7.).

For more information about it, see here.

To get back to the Admin page to open your site, see here.

For acrylian, the Installation and Upgrading page mentions a step for the site closing when running setup, but it does not have a step for what to do to open the site again. That might be worth adding.

Sorry, I don't understand your answer very well. #10 states "Once the files are uploaded, navigate to www.yoursite.com/zenphoto/ (if you have installed Zenphoto within the folder zenphoto). Setup.php will run. If it somehow does not, please navigate directly to www.yoursite.com/zenphoto/zp-core/setup.php. Info about the required permissions if you run into issues." I tried going to both /zenphoto/ and /zenphoto/zp-core/setup.php and neither one run setup.php. They just take me to a strange URL. Previously, even if the files didn't change, it would still show me the setup page.

For acrylian, the Installation and Upgrading page mentions a step for the site closing when running setup, but it does not have a step for what to do to open the site again. That might be worth adding.

There is a red note with a link in the admin tool bar on top of the page and also on the backend itself noting that the sitet is closed and where to change it. Of course older releases don't have that.

You can also disable it in 1.6.x via the config file.

I tried going to both /zenphoto/ and /zenphoto/zp-core/setup.php and neither one run setup.php. They just take me to a strange URL. Previously, even if the files didn't change, it would still show me the setup page.

The "strange URL" you actually refer is the one you noted in the first post? That is the closed for mainttenance url in general although you seem to get weird long one. I have no idea why.

But you need to be loggedin with full admin rights to perform an upgrade. If you directly upgraded from 1.4.1.5 to the current 1.6.x we don't recommend that. as it is not tested.

I am definitely still logged in with full admin rights after closing. I can go directly back to the admin area without logging in. However, going to setup.php gets me that long closed url even though I am logged in. I didn't see any related errors in the error log.

I am not upgrading at all. I am just trying to access the setup script page without upgrading to verify it functions as expected. I have done this in the past when I got a notice that the database changed.

Do you try to access setup via the utility button "Run setup" or directly? I would not know why you would get that URL.

I don't see a utility button to run setup. I just go to the URLs specified, /zenphoto/ and /zenphoto/zp-core/setup.php. Maybe it will be better for me to do a totally new install and manually port the data over to the new install?

Well, going back to the site again just now I got the notice to run setup via link /zp-core/setup.php?autorun=gallery&xsrfToken=x and this worked, so I guess I'm back in business. No idea what happened before, but thanks for responding promptly!

Glad you were able to solve it!

I don't see a utility button to run setup.

That button would be on the main backend page (usually referred to as "Overview").

Also please remove any old files within /plugins/site_upgrade/. The maintenance mode was formerly a plugin and is core now for quite a while. While having custon maintenance pages there is still supported it might still be an issue with that old 1.4.x file not being compatible anymore. It's been a while so that I cannot remember details anymore…

Also just to be sure also please check your htaccess file to have "version 1.4.12" on top.

Man, that URL looks like it went down a serious rabbit hole! Glad that autorun link finally saved the day and got you back in business.

For future reference, those infinite redirect loops usually happen when the server gets confused by old, cached session data. Wait, actually, I think it's because the server is trying to read your code backwards from right to left, or, more likely, it's just those ancient 1.4 plugin files conflicting with the new core.

Definitely delete that old /plugins/site_upgrade/ folder so your site doesn't try to inception itself again!