[Plugin: WordPress MU Domain Mapping] Logged-in users unable to view "mapped" domain

When logged in as network administrator, visiting the, I guess, “mapped” domain (nomenclature isn’t clear at all – I’m refering to the TLD that I’m trying to use for the site, not the subdomain) the site either redirects infinitely, or displays an “invalid login key” message. Sometimes, it redirects for a while and then lands on that page. Sometimes it goes to that immediately.

I’m a very experienced developer and apache administrator, and I’ve followed the instructions so far, including all .htaccess modifications. I’m about to dump this and use mod_rewrite instead.

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