Neither timeline nor month view is showing up in node

Problem/Motivation

BAT, BAT_API, BAT_Event, BAT Booking, Fullcalendar - all installed.
I have an event with start and end date with an availability type and a BAT unit set.
Within a node I have a BAT Calendar Unit Reference with a unit and event type set.
Both units - in the event and in the node are the same.

I neither get the timeline nor the calendar view showing up.
The error in the console is:

drupal.js?v=10.3.2:64 Uncaught TypeError: $(...).once is not a function
    at Object.attach (bat_calendar_reference.js?v=10.3.2:13:17)
    at drupal.js?v=10.3.2:166:24
    at Array.forEach (<anonymous>)
    at Drupal.attachBehaviors (drupal.js?v=10.3.2:162:34)
    at big_pipe.js?v=10.3.2:153:10
    at big_pipe.js?v=10.3.2:184:3

Steps to reproduce

I used simpletest.me for a clean install with BAT and BAT API and depending modules and got the same problem.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes