Too many redirects / Endless loop error on Members Only pages when upgrade from Membership Lite to M

We're setting up Membership Lite right now on a site. It's going well overall, but when we try to upgrade to Membership Pro to take advantage of some of its features, we run into a major issue. All the pages that were restricted for registered members stop working. In IE, we just get an endless wait & then generic error, but in Chrome we get a message that there are "Too Many Redirects". From researching other forum posts, it appears this could be related to how the Membership "No Access" and theme "404.php" pages are interacting. Since the theme works fine with Membership Lite, though, not sure how to address the issue to make Membership Pro work.

Thanks for any insights you can provide.