Hi, can you please check browser console for error on that page? Should be error message with details what went wrong. Best regards, Nikola

I hope this helps 

bundle.js:2:866478

Uncaught (in promise) Error: Error: Request failed with status code 404     e https://my-domaine.com/wp-content/plugins/easy-appointments/js/bundle.js?ver=3.11.8:2     s https://my-domaine.com/wp-content/plugins/easy-appointments/js/bundle.js?ver=3.11.8:2     _invoke https://my-domaine.com/wp-content/plugins/easy-appointments/js/bundle.js?ver=3.11.8:2     A https://my-domaine.com/wp-content/plugins/easy-appointments/js/bundle.js?ver=3.11.8:2     Kt https://my-domaine.com/wp-content/plugins/easy-appointments/js/bundle.js?ver=3.11.8:2     c https://my-domaine.com/wp-content/plugins/easy-appointments/js/bundle.js?ver=3.11.8:2

Looks like there is no endpoint for handling Vacation requests. Do you have any caching engine on your site? Do you block WP API part on your site? Best regards, Nikola

I have this plug-in
WordPress Native PHP Sessions
https://dev-acted-appointment.pantheonsite.io/wp-admin/plugin-install.php?tab=plugin-information&plugin=wp-native-php-sessions&TB_iframe=true&width=600&height=550
This is the only one that can work with cache.
I deactivated it, but the problem remained.