“Trying to perform an illegal action” error

When some people submit an event on our public “suggest an event” page, they receive an error that reads, “Trying to perform an illegal action.” and the event does not get submitted. When I add an event through the same page, it goes through successfully. Does anyone know why this is happening, and how to fix it? Is it a permissions issue, and if so, what needs to be changed?

Thank you!

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