Membership "Stranger" level not working

I had started with Membership Lite, and successfully configured a "visitor" access level for strangers to allow unauthenticated users to see (most of) the site. After upgrading to the paid version of Membership this isn't working - "strangers" are able to see the pages in the menu, but not any page content. On the home page I am also getting this error:

Warning: Cannot modify header information - headers already sent by (output started at /home/benfineman/ in /home/benfineman/ on line 881

I have disabled all other plugins. And confirmed that the "visitors" level is active. What can I do next?