I tried to set up a sub-site on an existing WordPress 3.5 site. The admin let me create the site, but when I tried to visit the sub-site’s dashboard, I got a “too many redirects” error from the browser.

I googled for hours. I tried many, many things. In the end, this particular version of .htaccess worked for me. YMMV. Good luck.

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) /$2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ /$2 [L]
RewriteRule . index.php [L]