I have exactly the same problem.
I can add users to the calendar and invite them, except for one specific user. This user has had a lot of issues connecting to her account, so I had to recreate the user twice. However, in the user list she appears only once. Now, when I try to invite this user to an event, it never works.
When I add the user to the event, then click update, this file is being sent in an Ajax request:
https://example.org/remote.php/dav/calendars/Admin/personal/FC5D89A9-771B-4D48-8804-3E73CE69190B.ics
But the request returns a 404.
When I download that ICS file, I don’t see the name of my attendee in there, but only the names of the other attendees, that I had added previously to the event.
In the JS console of my browser I see a JS error:
vue.runtime.esm.js:1888 Error
at XMLHttpRequest.n.value.regeneratorRuntime.mark.regeneratorRuntime.wrap.e.abrupt.a.onreadystatechange (request.js:353)
I see on that line that the error is NetworkRequestClientError, which seems to be indicating a 404 for the XHR request using the .ics file. But why is this .ics resulting in a 404, while it does not happen when I add other attendees, stays obscure to me.
Is there a way to get more precise errors?
I read in this thread that the issue might be linked to non unique email addresses. Therefore, I tried to change this user’s address to another email address. But I get the same error. Same if I try to change her username.
I tried to recreate the user: the problem persists. But I can still invite every other user to the event… 
Usernames have no spaces or other weird issues.
Sending notification emails is activated in settings.
Sending notification emails works as intended for every other user invited to the event.
I’m using NextCloud 19.0.3 and Calendar 2.04.
Initial settings:
- AppVersion: 2.0.4
- EventLimit: true
- FirstRun: true
- ShowWeekNumbers: false
- ShowTasks: true
- ShowWeekends: true
- SkipPopover: false
- SlotDuration: 00:30:00
- TalkEnabled: true
- TasksEnabled: false
- Timezone: automatic