For your information: I have installed the latest Version of Custom Contact Forms and the latest Version of WordPress.
In the console I find this errors:
[Error] ReferenceError: Can’t find variable: WP_API_Settings
global code (wp-api.js:920)
[Error] TypeError: undefined is not an object (evaluating ‘wp.api.models’)
(anonymous function) (form-manager.min.js:1:5766)
global code (form-manager.min.js:1:16291)
[Error] TypeError: undefined is not an object (evaluating ‘wp.ccf.forms.findWhere’)
fetch (form-cpt-preview.min.js:1:580)
preview (form-cpt-preview.min.js:1:112)
(anonymous function) (form-cpt-preview.min.js:1:1227)
forEach (load-scripts.php:29:2352)
(anonymous function) (form-cpt-preview.min.js:1:1147)
global code (form-cpt-preview.min.js:1:1236)