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/kindredcam.com/wp-content/themes/SimplePress/epanel/custom_functions.php:374) in /home/benfineman/kindredcam.com/wp-includes/pluggable.php on line 881

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

Thanks
Ben