I haven't upgraded the version of PHP since I started my web host account.

I ran chmod to make sure the permissions for everything in the Zenphoto installation is the standard 775, except of course for zp-data/ which is the recommended 0600.

No effect.

The bit about RLimitCPU, mod_perl, and suEXEC are kind of over my head, but I'm guessing if it's all in Apache, then issues with these would also cause problems for any other sites I'm hosting -- yet my other sites on this account, including one which also uses Zenphoto, don't seem to be affected.

I have no idea and would guess some server configuration error. I sadly have no knowledge of that as I leave that to hosts. I can only say that I have Zenphoto successfully installed on half a dozend standard shared hostings.

Did you try to reupload all files? Maybe something went wrong with the upload.

Last rather clueless idea: In the errors I see https. Are you running the site in that mode? Did you check that option on the backend and is the domain really setup for https? Technically a https/https://domain.com are separate - on my host I need to explicity enable the https one to make it work.

I actually have the server protocol set only to "secure admin" but I've noticed lately that the gallery has been running in https anyway, maybe because I have the HTTPS Everywhere browser plugin (though it didn't always do this for the front end) or because I'm usually logged in when trying the Zenphoto site.

Using an unencrypted connection doesn't change the behavior, though, and the server logs show the same errors for regular http.

If I were you I would start a triage of the problem.

Simplify your site as much as possible (e.g. remove that browser plugin) and see if things work. If so then start adding things back until it fails. Then you will know what is causing the issue.

I ended up re-reinstalling to a different directory and moving all the albums to the new location. Before, the Zenphoto site had been at ~/MYDOMAIN.tld/media/zenphoto/. Now it's just at ~/MYDOMAIN.tld/media/.

Everything seems to work fine now, just as it should. Just like it used to. I don't know why this would make a difference, and I'm maybe a little disappointed at not solving the mysterious server problem, but this went on for too long, and I guess I don't care anymore. At least it works now.

Thanks for all your help, guys.

Glad you have it working. Guessing from the solution your problem was most likely file/folder permissions.

Chmod 777 worked for me too. I think that one is the easiest.

You hopefully don't use that on a public site…