Login problem – Cookie error message (And other problems)

I’m having the same problem reported in a few other threads on wordpress.org, and I’ve tried the solutions listed but none worked. I’ve tried modifying the wp-login.php file, the .htc file, and the wp-config.php file … no luck. I’m posting this in the plugin support as well in case anyone here has any ideas? (Please?)

I have a multisite installations hosted on Dreamhost, and I’m using this domain mapping plugin. I have two outside domains mapped to the network, one which was set up around the start of October, and another which I set up today. The older one works fine, and lets me log in. The one I just set up is giving me an error when I try to log in (user and password is definitely correct): “ERROR: Cookies are blocked or not supported by your browser. You must enable cookies to use WordPress.”

Modifying wp-login.php by commenting out lines 744-746 gets rid of the cookie error, but still does not let me log in. When attempting to log in with incorrect credentials it gives the appropriate error message, but the correct credentials simply redirect back to the login screen and don’t give any access to the admin side of things.

I have no clue why the old site at another mapped domain lets me log in just fine, while the new one does not. Has something changed in the way mapped domains are set up since then? (I do think the 3.7.1 update was in between?) It would make more sense if both outside mapped domains stopped working… but why just the new one?

P.S.: These are the other threads I’ve found and the directions I’ve tried with no success:
http://wordpress.org/support/topic/cant-resolve-error-with-cookies-preventing-login-to-admin-on-second-website
http://wordpress.org/support/topic/cookies-are-blocked-or-not-supported-by-your-browser-1
http://wordpress.org/support/topic/cookie-error-site-not-letting-me-log-in

http://wordpress.org/plugins/wordpress-mu-domain-mapping/