Internal Server Error! Please try again.

Installed and activated the plugin on a multisite blog.
I can create a quiz and add pages to the quiz. But I cannot create any questions and it doesn’t seem to save it. Clicking that button results in “Internal Server Error! Please try again.”

Screenshot: https://www.dropbox.com/s/ynzhlqin88zgvnm/Screen%20Shot%202018-10-18%20at%203.21.47%20PM.png?dl=0

I suspect a permissions issue somewhere.

The page I need help with: [log in to see the link]