I've been searching for someone else on the forums with this problem, and come up empty.
Using plain text editor for *any* editing on my cart produces the error:

You don't have permission to access /shop/admin/define_pages_editor.php on this server.

I've tried CHMODing the permissions on the file, to no avail.
I'm fully up to date on my Zen version (1.3.7.1), and moderately modified. PHP 4.47 (awaiting an upgrade), Apache/Linux server.

This is a fairly recent occurrence, but I can't tie it to a specific upgrade/mod.

I'm out of ideas (and doing the files offline/locally is a PItA). Any advice will be most appreciated!