Can't add custom category to calendar

I’ve just set up Nextcloud AIO on a Docker install and have set up accounts and stuff, and am trying to use the calendar. Any time I attempt to add a new category under “Search and add categories” when adding a calendar event, clicking on it just has it stay there, without adding the category, and with no way to get the dropdown to go away. Trying to then add the event just has the panel show a loading spinner forever and never actually does anything.

Basically, are calendar categories bugged or something? Or is there some other way I’m supposed to be doing this? Because I can’t figure out a way to use them. (Also, adding an existing category is fine, but I can add it multiple times but not remove any added ones - what’s up with this, too?)