‘undefined’ popup when trying to book

I’m intermittently getting an ‘undefined’ dialog popup when trying to book an appointment. The form works fine for two or three bookings and then the popup occurs. On those occasions, a slot is reserved but the appointment is not populated with the bookers personal details.

The console shows the following error when the problem occurs:

Failed to load resource: the server responded with a status of 400 ()

..with a link to a admin-ajax.php url.

I’ve tried deactivating all plugins, switching the theme to Twenty Twent-three and replacing the htaccess content. But so far cannot determine what is causing the intermittent error.

Any help would be greatly appreciated.

Thanks!

Chris