Mail invitation - There was an error updating your attendance status.

Steps to reproduce

1. Create an appointment
2. Add attendee to meeting -> mail is created
3. Click accept button in mail

Expected behaviour

Status of attendee should be updated in the calendar view.

Actual behaviour

The following error is shown:
There was an error updating your attendance status.
Please contact the organizer directly.

Already tried

I already found this post with the same problem and tried to use another mail address for my Nextcloud account without success. There is currently no user sharing the exact same mail address.
https://help.nextcloud.com/t/solved-cannot-accept-calendar-invitation/50154

Server configuration

Operating system:
Ubuntu 16.04
Web server:
Apache
Database:
MySQL
PHP version:
7.2.12
Server version: (see your admin page)
15.0.7
Calendar version: (see the apps page)
1.6.5
Updated from an older installed version or fresh install:
Yes, updated since ownCloud 8
Signing status (ownCloud/Nextcloud 9.0 and above):

No errors have been found.

List of activated apps:

Enabled:
- accessibility: 1.1.0
- activity: 2.8.2
- admin_audit: 1.5.0
- bruteforcesettings: 1.3.0
- calendar: 1.6.5
- cloud_federation_api: 0.1.0
- contacts: 3.1.1
- data_request: 1.2.0
- dav: 1.8.1
- deck: 0.6.1
- federatedfilesharing: 1.5.0
- federation: 1.5.0
- files: 1.10.0
- files_markdown: 2.0.6
- files_pdfviewer: 1.4.0
- files_sharing: 1.7.0
- files_texteditor: 2.7.0
- files_trashbin: 1.5.0
- files_versions: 1.8.0
- files_videoplayer: 1.4.0
- gallery: 18.2.0
- groupfolders: 3.0.0
- lookup_server_connector: 1.3.0
- notifications: 2.3.0
- oauth2: 1.3.0
- password_policy: 1.5.0
- provisioning_api: 1.5.0
- quota_warning: 1.4.0
- sharebymail: 1.5.0
- tasks: 0.9.8
- theming: 1.6.0
- twofactor_backupcodes: 1.4.1
- twofactor_totp: 2.1.2
- twofactor_u2f: 2.1.3
- updatenotification: 1.5.0
- workflowengine: 1.5.0
Disabled:
- comments
- encryption
- files_external
- files_retention
- firstrunwizard
- logreader
- nextcloud_announcements
- serverinfo
- support
- survey_client
- systemtags
- user_ldap

Nextcloud configuration:

{
"system": {
"instanceid": "***REMOVED SENSITIVE VALUE***",
"passwordsalt": "***REMOVED SENSITIVE VALUE***",
"secret": "***REMOVED SENSITIVE VALUE***",
"trusted_domains": [
"cloud.xxxxxxx.xxxxxxx"
],
"datadirectory": "***REMOVED SENSITIVE VALUE***",
"overwrite.cli.url": "https:\/\/cloud.xxxxxxx.xxxxxxx",
"dbtype": "mysql",
"version": "15.0.7.0",
"dbname": "***REMOVED SENSITIVE VALUE***",
"dbhost": "***REMOVED SENSITIVE VALUE***",
"dbport": "",
"dbtableprefix": "oc_",
"dbuser": "***REMOVED SENSITIVE VALUE***",
"dbpassword": "***REMOVED SENSITIVE VALUE***",
"logtimezone": "UTC",
"installed": true,
"maintenance": false,
"loglevel": 2,
"theme": "",
"trashbin_retention_obligation": "auto, 14",
"mail_domain": "***REMOVED SENSITIVE VALUE***",
"mail_from_address": "***REMOVED SENSITIVE VALUE***",
"mail_smtpmode": "sendmail",
"mail_smtpauthtype": "LOGIN",
"updater.release.channel": "stable"
}
}

Are you using external storage, if yes which one: local/smb/sftp/...
no

Are you using encryption: yes/no
no

Are you using an external user-backend, if yes which one: LDAP/ActiveDirectory/Webdav/...
no

Client configuration

Browser:
Firefox , Chrome

Operating system:
Windows 10, macOSX, iOS

CalDAV-clients:
iOS

Logs

Web server error log

Nothing

Log file (data/nextcloud.log)

Nothing

Browser log

Nothing