Invitation emails not sent for shared calendar

[armaccloud]

Steps to reproduce

1. Make sure you have either sendmail or SMTP configured on your nextcloud instance
2. Make sure you have two user account on the same Nextcloud instance - User1 and User2
3. Create a calendar as user1 and share this calendar with user2
4. Open the calendar as user2
5. Create an event as user2 and add an attendee

Expected behaviour

A calendar invitation email is sent to the attendee.

Actual behaviour

A calendar invitation email is not sent to the attendee

Note: When deleting the event, a cancellation email IS sent to the attendee! So the mechanism does work, just not for invitations.
Note: When creating an event as user1 (owner of the calendar), the email invitation IS sent to the attendee.

Calendar app

Calendar app version: 2.1.2

Client configuration

Browser: Firefox 83.0

Operating system: Pop_!OS 20.04

Server configuration

Hosted at hostiso/cloudamo

Nextcloud Version: 20.0.2

Updated from an older installed version or fresh install: Updated

[tcitworld]

Duplicate from #2055, see nextcloud/server#15676 and #2145 as well.