We are running: 
Moodle version (4.1.6)
Scheduler version 4.0.0 (build 2023052301)
 
The issue we are having is that when we click the "Add another student" button in an appointment we just get:

 
 
rather than:

 
This bug only seems to affect us on slots that are on later pages, sometimes this happens on page 2 of the slots, and on other schedulers its happening on page 3.
 
It seems llike the XHR callback that provides the data to fill the student selection dialog box is not returning with the data. 
 
Any clues on how to further debug this would be really useful.