Okay, I just enabled access to pages on the "Add/Edit Membership Level" page, and it seems to have solved the problem.
FYI, I do think the text instructions are potentially misleading. It says in the "Granted Access to" tab that "You can globally turn on or off access to content (eg. all posts, pages, comments) for this membership level here. More customized content protection (eg. access to certain posts or pages) can be done via the Manage Content Protection menu".
I read this as saying that if I wanted to give access to ALL pages I could set that option here, but if I wanted to give access to only a subset of pages I should use the Manage Content Protection menu. I didn't read this as implying that I could only give access to a subset of pages if I first gave access to all of them.
But thanks very much for the reply! I'm thrilled with your plugin so far!