Sorry, security error

The form works fine on my macbook, even when logged out of admin, but when i try to use the form on my phone I get an error when i complete the form that says “Sorry, security error” and a client has also reported they got the same error message – also, I haven’t had any submissions for the last four months so maybe this is an issue for everyone except me!!

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