I can’t recreate this issue and since it’s a free plugin, I can’t trouble-shoot everyone’s install to see what other plugin might be interfering. Try it on a clean install or disable all other plugins and then try.
Check your database and ensure all of the fields are filled in for the wp_options table entry “slp_settings” that holds the field data for the settings page.
a:4:{s:6:"enable";s:1:"1";s:12:"redirect_url";s:34:"https://youtu.be/dQw4w9WgXcQ?t=2m6s";s:8:"question";s:3:"dog";s:6:"answer";s:5:"breath";}
I just logged into all of my sites, some 3.5.1 and others a 3.6 RC version.
If you really want this solved, we’ll have to do this as a service or you can fork the plugin on GitHub and edit the code (or edit the code in your plugin folder).