The problem appears to be at line 468 of securitycheck.class.php. For some reason this line is returning false:
if(defined(WP_CONTENT_DIR)){
It checks to see if there is a WP_CONTENT_DIR define in your wp-config.php. An alternate way to check (which fixed the issue for me) is this:
if(constant('WP_CONTENT_DIR')!=NULL){
Altering plugin files is a short term fix though – it will be overwritten with the next update. Hopefully the plugin author will fix the issue before then.
