Yep, that's the issue.
D'oh! .htaccess rewrite path!
.htaccess:
#### !!! Change this to the web path (eg: http://www.yourwebsite.com/photos --> /photos) !!!
Okay, I fixed that and now the first error no longer appears. Now it's only when tinyZenpage gets as far as loading the images from the gallery.
[Wed Jan 07 06:38:04 2009] [error] [client x.y.z.a] File does not exist: /home/mgjk/xxx/zp-core, referer: http://xxx.yyy.zzz/photos/zp-core/plugins/tiny_mce/plugins/tinyzenpage/tinyzenpage.php?album=Berlin
[Wed Jan 07 06:38:04 2009] [error] [client x.y.z.a] File does not exist: /home/mgjk/xxx/missing.html, referer: http://xxx.yyy.zzz/photos/zp-core/plugins/tiny_mce/plugins/tinyzenpage/tinyzenpage.php?album=Berlin
This repeats for each image.
According to the the dialogue in tinyZenpage, it's 1.0.1, Zenphoto tells me that at the bottom of the main page, it is 1.2.2, and that the Zenpage plugin is 1.0.1.
I upgraded Zenphoto in-place before installing Zenpage. It used to be something in the 1.1 series. I can't recall. The rewrite bug was probably always there, it's just that it didn't actually cause any problems.
I'm pretty sure Zenpage's setup.php ran when I first installed it, no problem. I've tried running it manually since, but it just seems happy that everything is already set.
Since I messed up the .htaccess rewrite config, could the setup.php have stored any path information in the MySQL DB? Any other places to look?