Thanks, Hugh, but I'm afraid that's not it.
I tried your suggestion, and renamed to the htaccess file to oldhtaccess, but I still receive the 303 message.
For what it's worth, the URL you reference (index.php?option=com_users&view=login&Itemid=204) is a Main Menu link set to be the home page for the site. The URL lands on a Fabrik form available to registered users. Prior to upgrade, when users landed on the URL & they weren't logged in, the site would ask them to log in. When they did, the Fabrik from would resolve.
If I set the home page to a different page on the site, then the site will resolve just fine. It's only when I try to set the home page for the site to resolve to the Fabrik form that I have a problem.
Any other additional suggestions?
Yours,
Brent