Getting a 500 internal server error ONLY on wp-admin/options-permalink.php

I have access to the admin panel, and everything works fine. I can click settings->reading and change anything I want to. But if I click settings->permalinks I get a 500 internal server error. Here's what I've tried:

I deleted the .htaccess file - nothing changed. I edited the .htaccess (because I am migrating this site to a new domain) and now I have all the page links working on the front end AND all the admin links EXCEPT permalinks. In other words, the front-facing portion of the website works. All of the admin features work. But if I click settings->permalinks, it takes me to /wp-admin/options-permalink.php but loads my theme's index.php template. Why?

I also re-installed all WordPress core files, to no avail.

Anyone have any ideas?

Here's my current .htaccess file:

# BEGIN WordPress
IfModule mod_rewrite.c
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
/IfModule

# END WordPress