After diving into tons of support threads I found a couple that alluded to needing to turn off authentication profiles - this one (though related to a different error) and this one (which I think is inaccurate as I was getting “user does not exist” for an email address that IS a Zoom account) in particular.
Anyway, I’ve now managed to get this working by adding
"meeting_authentication" => false
to the API call that creates the meeting, i.e. disabling “Only authenticated users can join”.
Is this correct? It seems quite illogical that in order to turn on registration you have to turn off authentication.
This suggests that if you use registration to pre-authorise meeting participants then registering someone without a Zoom account means they can join the meeting without an account. If so, that’s fine - just want to get this straight in my mind and make sure I’m doing the right thing and not compromising meeting security in any way.