WP Multisite Admin Login issues

I have a WP multisite with a base domain (ex. wp.mysite.com). There are ~8 different sub-sites on this multisite.

I have no problems logging in on sites with their own domain (ex site1.mysite.com, site2.mysite.com, site3.mysite.com).

Trying to log into wp.mysite.com/wp-admin or wp.mysite.com/site4/wp-admin redirects (most of the time) back to wp.mysite.com/wp-login.php. Logins succeed about 20% of the time, and it doesn't matter who is logging in (we have some users that login using Active Directory credentials & some who log in using WP username/pw).

This just started happening on an install of WP that's been active for a few years. No major changes have been made. I've attempted disabling plugins & going back to the base theme. A clone of the site (on a different domain) doesn't exhibit the same issues.

Could this be a domain issue instead of a WordPress issue?

We're currently running WordPress 3.9.1. Using plugin Active Directory Integration for the AD Credential Logins (tested with and without this plugin active with the same results).

Let me know if I can provide any additional information.

Melanie, Valencia College OIT Webservices