After upgrading from 2.4.1 to 2.5.7 the modal ‘insert’ buttons in the panel stopped functioning. They simply don’t react when clicked upon. This happens with the ‘mail’ and ‘link’ button, customer unhappy, customer wants buttons 
Happened both on the production server and my localhost.
I upgraded by simply replacing both the 2.4.1 Kirby and Panel folder with the 2.5.7 folders.