I'm running Wordpress (v3.5.2) in network mode.
Any of the sites which try to access /wp-admin produce a browser error saying there is a redirect loop. A redirect checker tool says the page returns a 302 temporary redirect to the same URL.
I have uploaded a default .htaccess file and the same thing happens.
In cPanel, I have checked Domains > Redirects, and there is no such redirect listed.
I have renamed /plugins to /plugins2, and the issue remains.
In the wp_options table, the site_URL is set to www.example.com, which is correct. This matches the DOMAIN_CURRENT_SITE parameter in wp-config.