But it had not been updated in some time 'cause, you know, it was working fine.

You should really upgrade regulary for bugfixes and especially securitty fixes. Also doing this reguliary causes less work because of changes at the time.

Sadly there is no really reliable way to check for htaccess and ours requires the cURL library. So the report may be wrong and it is working actually. If you enable the modrewrite options you need to make sure the .htaccess file is there. Within zp-core/file-templates there is the template file. Rename it and best re-run setup.

So if the fresh install works, please check for differences with the older install. Especailly any third party plugins within /plugins or outdated third party themes in /themes. If your theme was customized, remove it for now and use a standard theme. Just in case these interfere which may be the cause if you didn't upgrade for some time.

I did it!
Just to wrap this up so no one is left wondering…
After creating the brand new database and zp install, and testing with one album, I summoned my courage and copied the zp-albums and zp-images tables from the old database into the new one. Then I copied all the albums and images into the new zenphoto directory and so far as I can tell I have all the legacy captions.
What a super relief. Thanks again for your patient help throughout this time of frustration and uncertainty.
And of course, thank you for zenphoto in the first place!
Steve

Great, thanks for the follow up.

Just to note: The correct order is always folders first, then database as documented here: https://www.zenphoto.org/news/moving-and-changing-existing-installations/

If you had any kind of visitor in the meantime, Zenphoto would check the filesystem with the database and updated db after the filesystem, not the other way round.
For those albums/images requested by that visitor, so not for all at the same time, as the discovery happens "on request".

Right. My earlier post was misleading. I did move the images and folders into place before tinkering with the database. Dumb luck I guess!