Sorry, your login session has changed/expired... please try again

Hello,
I am consistantly getting this error and cannot figure out why, I am narrowed it down to iphones, but cannot see a way to fix or stop it from happening. From PC, or any android device the issue never happens and the contact form submits no problem.

Any thoughts?
Note: This is a form on the contact page so people submitted do not need to login.

Greatly appreciate any help on this please.