Unable to subscribe to list – “You do not have permission for this request”

Hi guys,

For some reason, the opt-in forms are not allowing people to subscribe to my mail lists. The following error comes up:

“You do not have permission for this request /wp-admin/admin-ajax.php?callback=jQuery112406964683540260974_1537864620591”

I’ve not been able to get it working even after disabling my security plugin. I don’t know if it’s related to an update or some other thing going on? Could you point me in the right direction, please?

Thank you very much!
– Luciano

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