Event Invitation - Calendar App

On several Nextcloud instances I have a problem with the Calendar application.
When you invite an outside person (by email) the person never receives the invitation.
I confirm that the mail server is functional and that the “Calendar server” options in the administration section are activated.

During an internal invitation, the latter is well received on the mailbox. However, even after accepting the event, it does not appear in the invitation but appears accepted on the organizer side.
This is currently happening on 4 different instances.
Nextcloud Version: 23.0.2
Calendar Version: 3.2.1

Here are the logs (identical on all instances):

[no application in context] Warning: Invalid force_event_alarm_type data provided to provideInitialState by calendar

GET /apps/calendar/
from per user at 2022-03-16T09:04:39+01:00

is the calendar shared?

Hello @ChristophWurst,
Yes it is a shared calendar between users of the same Nextcloud instance.

Mail invitation, when using shared calendars, has been disabled years ago, if I’m informed correctly. I’ve read that this violates the CalDAV standard.

But I’m surprised that internal invitation mails work for you. It does not work for me.

I’m also looking for a workaround for this since 8 months:

EDIT: Creating events in shared calender doesn't trigger email has the answer to this question (but not a solution, which I’m still searching for).

For my case the invitations by email work now but when the latter is accepted the event is not created. But the event of origin (of the person who realizes the invitation) the status of the participants is well updated.
This concerns internal invitations.