Guestbook submission needs third-party cookies enabled.
Enable cookies for this site, then reload this page and submit again:
Chrome -> Settings -> Privacy and security -> Third-party cookies -> Allow third-party cookies
Edge -> Settings -> Cookies and site permissions -> Manage and delete cookies -> Allow third-party cookies
Firefox -> Settings -> Privacy & Security -> Enhanced Tracking Protection -> choose Standard
Safari -> Settings/Preferences -> Privacy -> turn off Prevent cross-site tracking